Understanding the consequences of inbreeding has important implications for a wide variety of topics in population biology. However, most studies quantifying the effects of inbreeding are performed under artificial farm, greenhouse, laboratory or zoo conditions. Although several authors have argued that the deleterious effects of inbreeding (inbreeding depression) are likely to be more severe under natural field conditions than in artificial experimental environments, these arguments are usually speculative or based on indirect comparisons. We quantified the effects of inbreeding on fitness traits in a tree–hole–breeding mosquito (Aedes geniculatus) under near–optimal laboratory conditions and in three natural tree holes. Our index of fitness (R0) was lower in the field than in the laboratory and declined due to inbreeding in both environments. However, we found no significant interactions between inbreeding depression and environmental conditions. In both the field and laboratory a 10% increase in the inbreeding coefficient (F) led to a 12–15% decline in fitness (R0). These results suggest that inbreeding depression will not necessarily be more extreme under natural field conditions than in the laboratory.