The genus Opuntia is native to central and southern Mexico. In this country it is a very important crop (^{Griffith, 2004}; ^{Casas and Barbera, 2002}) because the Mexicans, especially downtown, eat nopal (Opuntia young edible) for thousands of years. This plant is grown in more than 13 000 ha, but the village of Milpa Alta, south of Mexico City produces more than 40% of nopal (^{SIAP, 2013}). It is known in this area the existence of pests and diseases that reduce the production of this crop, mainly affecting the vigour of the plant, causing weakness, chlorosis and severe infestations that can cause death (^{Palomares-Pérez et al., 2010}; ^{CESAVEDF, 2011}; ^{Palomares-Pérez et al, 2012}). However, in Mexico there are no formal studies to identify the organisms associated with growing nopal, Opuntia ficus-indica L. (Miller) and the effects or damage they may cause to the plant, there are no studies to know their distribution in the crop, for this reason, the aim of this paper is to present the different nematodes present and identify those that could possibly cause some damage to the crop.

Fieldwork was conducted in the village of Milpa Alta, Distrito Federal, Mexico, located between 19° 04'- 19° 12 north latitude and 99° 08' - 99° 57' west longitude, 2 200-3 000 m. The sampling method was based on monthly collected soil samples from April 2011 to September 2012. Due to the diverse mountain area, and operability for sampling, the collected soil was performed using a simple stratified sampling Based on altitude and directions. The samples were extracted with an agricultural digger of square handles placed 5 cm from the stem of the plant and 15 cm deep. The study area was divided into four categories according to altitude range: AI) 2 950-2 750; A-II) 2 750-2 600; A- III) 2 600- 2 470 and A-IV) 2 470 a 2 320 m. The sites in the study area were duly georeferenced with a satellite geoposicionador (Garmin GPS 12 XL Olathe, KS, USA). In this way a total of 12 plots as sampling sites were chosen. In each plot subsampling was performed under the five “golds” methodology, with roots corresponding to 1 kg each, homogenized and a final sample of 1 kg was obtained.

Nematode extraction was performed using each sample of 250 cm3 of soil to quantify cysts by Fenwick flotation technique (^{Southey, 1985}) and for threadlike nematodes we used 200 cm3 of soil by sieving and centrifugation method (^{S'Jacob and Bezooijen, 1984}).

Filiform extracted specimens were fixed in 4% formaldehyde hot and transferred to anhydrous glycerin (^{Seinhorst, 1959}; Seinhorst, 1962). Specimens mounted permanently on slides were examined, for which the method of the wax was used (^{S'Jacob and Bezooijen, 1984}). For the cyst nematodes, the morphological characteristics were determined using a micrometer analysis.

Once assembled, they proceeded to their identification with the support of specialized literature: the keys to genera and species of Heteroderidae family of ^{Sosa-Moss (1997)} of cyst nematodes of ^{Mai and Lyon (1975)} and cyst and filiform of ^{Zuckerman et al. (1990)}, for Tylenchida of ^{Siddiqi (2000)}; for Aphelenchida of ^{Hunt (1993)}; and for Dorylaimida of ^{Jairajpuri and Ahmad (1992)}.

A total of 11 kinds of pathogenic nematodes were identified on the rhizosphere of O. ficus -indica distributed in the coordinates and altitudes above (Table 1).

Table 1 Coordinates and elevation where different nematode genera identified in Milpa Alta, Mexico City were collected. 

1) Aphelenchus; 2) Criconemella; 3) Dorylaimida; 4) Helicotilenchus; 5) Heterodera; 6) Hoplolaimus; 7) Longidorus; 8) Nacobbus; 9) Paratylenchus; 10) Tylenchorhinchus; 11) Tylenchus; y 12) Heterodera (Quistes).

Of the species found, Tylenchorhinchus sp., apparently showed a higher distribution by appearing more often in ten of the 12 sampled sites, followed by Aphelenchus sp. with a presence in seven sites Criconemella sp., and Cactodera cacti (^{Filipjev and Schuurmans-Stekhoven, 1941}; Krall and Krall, 1978) with presence in six of the sampled locations. Of these species which represents a higher danger to the area is C. cacti species probably originated in Mexico, reported as a major pest of cacti, including Opuntia (^{Adam, 1932}; ^{Filipjev and Schuurmans- Stekhojv, 1941}; ^{Ebsary, 1986}). ^{Borrego and Burgos (1986)} mentioned that C. cacti is mentioned as one of the species of nematodes that could cause problems nopal, ^{Rafiq (1986)} on the other hand, states that found specimens of C. cacti in the root system of cacti such as Disonopal ackermannii (Lindl.) Barthlott, Cereus spinulosus (DC) Britton and Rose and other species of the family that are used as ornamental in European, Asian and American. Similarly, ^{Evans and Rowe (1998)} mentioned that, the species Cactodera are found in members of Cactaceae, Amarantaceae and Chenopodiaceae in Durango and sour tuna (Xoconostle) in Hidalgo (^{Baldwin and Mundo-Ocampo, 1992}; ^{Montiel et al., 2001}). The presence in Mexico of C. cacti has great economic importance due to its great ability to survive as a cyst, which allows on the one hand, spread from region to region and from country to country, and otherwise stay for many years as a primary source of inoculum infested land (^{Sosa-Moss, 1997}). Its pathogenicity has not been determined so far in the genus Opuntia, but the nematode may contribute to secondary infections by fungi and bacteria (^{Baldwin and Mundo-Ocampo, 1992}).

Longidorus and Aphelenchus have been reported on nopal in the Province of Santiago del Estero, Argentina, along with other nematodes causing problems of rot and stem breakage (^{Doucet et al., 2001})

The following eight genera: Criconemella, Dorylaimida, Helicotilenchus, Hoplolaimus, Nacobbus, Paratylenchus, Tylenchorhinchus, Tylenchus , are first reported in the rhizosphere of O. ficus-indica.