Composition, abundance, and main assemblages of holoplanktonic polychaetes were analysed in the Gulf of Tehuantepec. This Gulf, in Mexican waters of the eastern tropical Pacific, is a very productive system due to an upwelling during the winter. Zooplankton samples were taken over a station grid covering neritic and oceanic waters during March and November 1978. Most species showed their highest densities in the oceanic region. In the western Gulf, the area sampled in both seasons, mean polychaete abundance was higher in November (549.2 ind.1000 m(-3)) than in March (143.8 ind.1000 m(-3)). A regression tree analysis showed that pH and dissolved oxygen were the main factors affecting the total polychaete abundance over the study area during March. Identified families were: Lopadorhynchidae (88.8%), Alciopidae (6.6%), Iospilidae (2.8%), Tomopteridae (1. 1%) and Typhloscolecidae (0.7%). The dominant species, Pelagobia longicirrata, accounted for 86.5% of the total polychaete abundance. A Bray-Curtis analysis identified two main polychaete assemblages during March: "oceanic" and "neritic". Species richness and mean polychaete abundance were higher in the "oceanic" assemblage, and P. longicirrata, Plotohelmis capitata, Rhynchonerella gracilis and Lopadorhynchus henseni were the most frequent and abundant species in this assemblage. In the "neritic" assemblage, the iospilids Phalacrophorus uniformis and Iospilus phalacroides registered the highest relative abundance. It is suggested that the structure of holoplanktonic polychaete assemblages could be determined by the feeding habits of the species and their tolerance to the variability in environmental conditions
