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Crassostrea gigas is a bivalve mollusk of great commercial importance. In northwest Mexico its production is affected by mortalities whose infectious origin has not been clearly determined. This study determined the prevalence and intensity of infection with Perkinsus sp. in a culture of C. gigas in 2011-2012. The culture was performed in a long-line system with densities of 28 and 42 oysters/tray. A sample size of 30 oysters per month was determined. Diagnosis of Perkinsus sp. was done according to the protocols of the World Organization for Animal Health (OIE) for Fluid Thioglycolate Medium and PCR. Both methods were able to determine the prevalence of Perkinsus sp., which ranged between 3.3 and 40%. The infection intensity was low (levels 1-2) according to the Mackin scale. Cumulative mortality in densities between 28 and 24 oysters per trey was 4 and 6% respectively. The highest mortality of oysters and the highest prevalence of Perkinsus sp. occurred in September (2.7 and 16.6%) and October (1.5 and 23.3%), respectively, when the temperature was high. In conclusion, Perkinsus sp. was detected in a C. gigas culture in the Estero La Pitahaya, with moderate prevalence, low infection and increased presence in the warmest months of the culture cycle. [ABSTRACT FROM AUTHOR]