Seasonal variation in population density, reproduction and mortality was observed in an island population of the tropical forest rodent, P. semispinosus. Average density was 8.5 individuals/ha, with the maximum in Aug. being 1.3 times higher than the minimum in April. Reproduction continued year round, although breeding was most intensive in the dry season. The highest proportion of trappable young animals in the population occurred in July-Aug. Mortality was relatively constant throughout the year, but increased slightly from the dry season to late in the rainy season. Observed seasonal variation in density of spiny rats is caused by seasonal changes in reproduction intensity. The timing of reproduction is such that most young are born in the period of the richest food supply. This guarantees optimal food conditions for lactating females and weaned offspring.