Seasonal occurrence of parasitic copepods in wild bullseye puffer, Sphoeroides annulatus (Pisces: Tetraodontidae), was analyzed in conjunction with variation of biotic and abiotic factors. Eleven samples were taken between February 2007 and February 2008 in Santa María La Reforma lagoon (northwestern coast of México). In total, 337 fish was examined; 5 parasitic copepod species were observed, including Acantholochus zairae, Caligus serratus, Lepeophtheirus simplex, Pseudochondracanthus diceraus, and Parabrachiella sp. The most common species were L. simplex, P. diceraus, and C. serratus (overall prevalence, 59, 53, and 35%, respectively), which significantly varied in prevalence and mean intensity between sampling months. A seasonal pattern was only observed for L. simplex, with higher infection levels in the warmest month than in the coldest month. Statistical analyses indicated that the intensity of L. simplex was positively correlated with water temperature. There were no significant differences in prevalence and intensity of infection among female and male hosts. At the component community level, species richness ranged between 4 and 5 during most of the study period, and no seasonality was observed in the number of individuals, Shannon diversity index, evenness index, or the Berger–Parker dominance index. At the infracommunity level, 4 descriptors used (mean species richness, mean number of individuals, mean Brillouin's diversity index, and mean Berger–Parker index) varied significantly between sampling months, but no seasonality was observed, except for a slight increase in the number of individuals during the warmest month. A significant positive association was detected between number of individuals and water temperature and between host size and both species richness and number of individuals. This is the first account of the ecology of these 5 parasitic copepods. Although no significant association was detected between fish condition factor and the burden of parasitic copepods, given the high occurrence of the caligid copepod L. simplex, we suggest that this copepod could represent a threat for the culture of S. annulatus.