Abstract: The zooplankton and Physico-chemical characteristics of the lower Sombreiro river, was studied from August 2005-July2006 using standard methods. A total of 17 species from 6 taxonomic groups were recorded during the study period. The species with the highest number (248, 12.1%) was Paracyclops fimbriatus, while Mysis sp. had the lowest number (25, 1.2%). The highest number of zooplankton (262, 12.7%) was observed in May and the lowest (59, 2.9%) was recorded in September. Mysis sp. was not recorded between April and September. Halteria sp. was absent in July. Other species were observed in all the months. The family Copepoda was the highest (45.5%) group followed by Cladocera (23.4%), Euphausicea (10.3%), Protozoa (11.9%), Rotifera (7.8%) and Decapod Crustaceae (1.2%) was the least. More zooplankton was recorded in the wet season (1267), with 61.6%, than the dry season (790), with 38.4%. All species were observed in both seasons. In the wet season, Meganicliphanes norvegica had the highest record (159), while Paracyclops fimbriatus was the highest in number (95) for the dry season. Mysis sp. was the least recorded for both seasons, as 9 individuals were recorded for the wet season and 16 for the dry season. Station 1 had the highest record (631, 30.7%), while station 4 had the lowest (376, 18.3%). All the species were observed in all the stations, with the exception of Mysis sp. that was absent in station 4. The analysis of variance for the seasons and stations for number of zooplankton recorded showed that there was significant difference (F = 14.10, d.f = 604, p = 0.05) between the stations. There was, however, no significant difference between seasons and also none for the combined effects of seasons/stations.The distribution showed that station 1 differed significantly from the rest. Station 3 also differed significantly from others. Stations 2 and 4 did not differ significantly. Shannon-Wiener information function (H) showed that, the highest value was in January (1.085) and the least (0.875) in September. There was variability between the months. Equitability (Evenness) index (E), October had the highest value (0.994) and February, the lowest (0.882). The variations were minor through out the study period. Margalef�s species richness index (d) showed appreciable fluctuations. The lowest value (2.595) was observed in April. The highest value (4.212) was recorded in October. There was hardly any sharp variation from one month to the next. Simpson�s dominance index (c) showed that January had the highest value (0.183). August had the lowest (0.089). The trend was relatively smooth.The catch increased with higher conductivity, lower dissolved oxygen, relatively faster flow rates and more rainfall. The abundance seemed to decline with lower conductivity, higher turbidity, higher dissolved oxygen and slower flow rates. The temperature did not show much difference between the 2 extreme percentage collections. The correlation between zooplankton collected and turbidity was negative.