The conditions under which coffee is grown may affect its long-term sustainability through effects on soil fauna, including earthworms, which have a profound effect on abiotic and biotic properties and processes in natural and agricultural ecosystems. We examined differences in earthworm populations among sun (SUN) and shade (SHD) coffee and secondary forest (FOR) ecosystems, as well as soil order (Oxisols, Ultisols, Inceptisols) in western-central Puerto Rico. Ecosystem type significantly affected earthworm population density (individuals/m²), which followed the order: SUN (281) > SHD (125) = FOR (37). Earthworm biomass (g fresh weight/m²) was also significantly affected by ecosystem type, following the order: SUN (71) > SHD (34) = FOR (12). In general, neither earthworm biomass nor abundance was significantly related to litter quantity, litter composition or soil properties for any of the ecosystems studied. The absence of significant differences between SHD and FOR ecosystems suggests similarity in those environmental variables that constrain earthworm populations. The larger earthworm populations in coffee cultivated under sunlight may indicate that earthworms play a more important role in organic matter translocation and decomposition, nutrient cycling, and soil physical structure than in shade coffee or forest ecosystems.