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Background. Different species of Perkinsus spp. have been associated to mortality of commercial mollusks. Goals. The prevalence of Perkinsus sp. in the callista clam Megapitaria squalida was evaluated monthly (June 2015 to November 2016) to determine the relationship between the infection and size of clam. Methods. 5,000 clam seeds (5.73 mm height of the shell and 0.03 g of initial weight) were placed in a fishing refuge of Altata, Sinaloa. A total of 60 specimens were collected each month to evaluate their growth and parasite prevalence. The water physicochemical parameters (temperature, salinity, pH, dissolved oxygen, depth, and transparency) and the length, height, and width of the shell and final weight of M. squalida were obtained. Results. Ray’s thioglycollate fluid medium test (MFTR) detected spherical cells (10X) of dark color indicating the presence of presumed hypnospores of Perkinsus sp. during the first 17 months of sampling. The mean infection intensity was grade 2 (mild), while the maximum prevalence was 43.33% in the months of November (2015) and March (2016), with an average of 21.05%. None of the Perkinsus sp sample (n = 112) positively detected by MFTR confirmed the presence of the protozoan. There was no correlation (p >0.05) between the prevalence of the pathogen with all the parameters studied. Conclusions. The results suggest that M. squalida is not susceptible to Perkinsus sp. The present represents the first report of Perkinsus sp. In the callista clam M. squalida.