The annual reproductive cycle of the squalid callista Megapitaria squalida living in the Ojo de Liebre lagoon was analyzed from January to December 2001. The reproductive activity was analyzed qualitatively (through histological analysis), defining five gonad development stages (undifferentiated, developing, ripe, spawning and spent). A quantitative analysis was also conducted (through the gonadic index and oocyte diameter) to correlate the reproductive activity with water temperature and salinity. Male and female gonads developed and spawned in parallel through time. M. squalida displays a clearly seasonal reproductive cycle in the Ojo de Liebre lagoon, with a long reproductive activity period (January to August) that coincides with the rise in temperature, plus a well defined inactivity period (September to December) with 100% of undifferentiated or spent organisms and the onset of which coincides with the temperature decrease. M. squalida's reproductive cycle does not seem to be influenced by salinity variations in the Ojo de Liebre lagoon. The spawning size of M. squalida was determined at 92 mm SL at an approximate age of 2.2 y. However, some individuals started spawning at 64 mm SL. Marked differences were found in some aspects of the reproductive pattern of M. squalida compared with studies conducted in other sites, and the likely causes are discussed here.