The composition, distribution and abundance of the chaetognaths collected at Bahía de la Ascensión, eastern coast of the Yucatan Peninsula, were analyzed during three climatic periods (rainy, dry, and “northerlies”) of a year cycle (August, 1990–July, 1991). Highest mean chaetognath abundance was observed during the rainy period (209 ind./100m³), while lowest values occurred during the dry season (124 ind./100m³). Six species were identified, Sagitta hispida being the most abundant and widely distributed. It represented more than 95% of the total chaetognath numbers. Variations of the spatial distribution of the group throughout the year cycle seemed to be related to the three seasonal regimes. Cluster analysis (Jaccard Index) revealed two main faunistic assemblages clearly defined during the three seasons. The first one was representative of the resident fauna, characterized by the dominant S. hispida. The second one was a group of several oceanic species with a seasonally variable influence in the bay fauna. This influence generated a transitional assemblage, detected during the rainy and nortes seasons. These faunistic assemblages showed seasonally variable distributional patterns which were related to shifts of the hydrologic conditions and the wind regimes. As it considers three distinct seasons, the pattern described herein differs from those previously depicted for chaetognaths in other coastal systems.