Reproductive biology of the black land crab, Gecarcinus ruricola (Linnaeus, 1758), has been studied in the San Andres Archipelago in the Western Caribbean, on the islands of San Andres and Old Providence. Both sexes become mature at about 50 mm maximum carapace width, and very few specimens below this size take part in the annual breeding migration. In females the ovaries mature progressively from January to April, and laying occurs from May to July. There is no evidence that individual females lay more that once in a year. The breeding migrations were monitored in 2003 and 2004 by recording specimens crossing the coast road (running 50-100 m from the upper shore). On both San Andres and Old Providence migration was limited to specific stretches of the western shores. On both islands migration occurred from late April to July, intensity varying with time. Migration peaks were not consistent between islands or years, and could not be clearly linked to environmental factors. Migrating crabs were mostly female (> 80%), and a proportion were bearing ripe eggs: on San Andres ovigerous female predominated. Thus some females mated and laid eggs on the landward side of the coast road, others on the seaward side. Migrating females were on average larger than migrating males, in contrast to the situation in the non-migrating populations. Log egg number varied isometrically with log carapace width (slope ~3.0): for 70 mm CW the calculated fecundity was 85,000 eggs. Percentage reproductive investment, based on dry weight, ranged from 1.7 to 9.8%, with a mean of 4.95%. Recruitment was only observed once, with a mass return of megalops to Old Providence in June 2004. All evidence points to very irregular recruitment.