A field study was conducted in a sunlight coffee grove of Puerto Rico to determine the population dynamics of Mirax insularis Muesebeck, the coffee leafminer [Leucoptera coffeella (Guérin-Méneville)] larvae parasitoid. Coffee leaf samples were collected monthly starting at early spring (March) and finishing in late autumn (December) of 2005. The highest parasitism ratio (0.2095) was recorded during August, followed by that of December with 0.1971. The lowest parasitization ratio (0.0120) was obtained in November. The average parasitism percentage obtained during the study was 20.13. Although the parasitism ratio was very low from April to July, in August when the coffee leafminer (CLM) population peaked, M. insularis also increased exponentially. However, from September to November a dramatic decrease in the parasitism ratio was observed. Heavy rains occurred during that season decreasing the CLM population and thus affecting the viability of suitable larvae for parasitization.