Planktonic copepods are representative members of the coastal and estuarine biota. In Chetumal Bay, western Caribbean, the relevance of monitoring the copepod community at different scales is widely recognized. Zooplankton was sampled monthly during 3 seasons (northerlies, rainy, dry) during a year cycle (1997). The composition and distribution of planktonic copepods and their monthly and seasonal abundance was analyzed to assess the influence of environmental parameters on this community. Each seasonal period was characterized by distinct relative abundance values of the most abundant species. Acartia tonsa was dominant during the 3 seasons, with high relative abundances year-round (67-92%). The ANOSIM (Bray Curtis similarity analysis) showed significant differences of the community among seasons thus confirming a seasonal dynamics; a monthly significant variability was also observed. The PCA (Principal Components Analysis) showed that temperature and salinity explained 82% of the observed variability; additional analyses indicated that salinity best explains the distributional patterns of the species. Two distinct groups were detected, one representing the resident estuarine community with high abundances of A. tonsa, low abundances of A. lilljeborgi, low species richness, and sites of the inner areas of the bay and of nortes (January and November). The second group represents a foreigner coastal-neritic community with low abundances of A. tonsa, presence of A. lilljeborgi, high diversity of species; it includes sites of the dry and rainy seasons plus those of the external zone of the bay. The local dominance of A. tonsa in Chetumal is explained by the low concentration of predators and the low densities of A. lilljeborgi, mostly restricted to areas with higher salinity and also by its probable positive correlation with oxygen concentration which probably limits its populations in a hypoxic system like Chetumal.