The breeding behavior and sex ratio variation of the carrot squat lobster Pleuroncodes monodon were analyzed. Specimens were collected from experimental hauls in three acoustic pelagic resource assessment surveys during 2001 off the Peruvian coast. A total of 8517 individuals were differentiated according to sex. Males showed clear sexual dimorphism, while females were divided into ovigerous or non-ovigerous individuals. Size did not differ significantly between sexes (Kolmogorov-Smirnov test, P > 0.05); average sizes were smaller than 20 mm cephalothoraxic length. The sex ratio indicated that in winter males were more predominant than females (χ2 test, P < 0.05). Ovigerous females were observed during all the period of study, with major activity in spring, and population dispersion occurred during the breeding season.