Background.

The distribution of jellyfish includes diverse oceanic and coastal areas. Most of them have a certain affinity for different masses of water, which depends on their biology and their interaction with biotic and abiotic factors.

Objectives.

Contribute to the knowledge of the biodiversity of jellyfish (Cnidaria: Medusozoa) in the Mexican Pacific.

Methods.

The study material comes from zooplankton samples collected during March and October in 2003 by surface hauls, with a standard net in Banderas Bay, Jalisco-Nayarit, Mexico.

Results.

7966 jellyfish were determined in 32 species, 34 genera and 26 families, belonging to the classes Hydrozoa and Scyphozoa; Solmaris flavescens extend its geographical distribution in the northeastern Pacific, six species are new records for the Mexican Pacific and ten more are for the study area.

Conclusions.

This study expands the knowledge of the diversity and distribution of jellyfish, an important group of marine zooplankton in the Mexican Pacific waters; the results of this work indicate that more sampling effort is needed because this group has seasonal changes, and its distribution is both horizontal and vertical. Solmundella bitentaculata, Liriope teraphylla, Aglaura hemistoma and Rhopalonema velatum have biogeographic affinity in the circum-subtropical region of the oceans. Some of the specimens in the new records were found in young stages; therefore, further research efforts are required, with a multidisciplinary approach that integrates their taxonomy, evolution, phylogeny and life cycle.