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A random survey of 2,189 houses in rural and urban sections of central Chile was done to learn the frequency with which they were infested with Loxosceles laeta, a poisonous spider that causes severe envenomation of man. More urban (40.6%) than rural (24.4%) houses were infested, but a higher density of spiders was found in rural (11.9) than in urban (3.9) houses. Of the 5,449 spiders collected, 20.4% were immature; a ratio of one male to eight females was found among the adults. The high proportion of infested houses strengthens the assumption that this species is essentially domestic and explains why its envenomation of man occurs principally indoors. The presence of these spiders is related to the characteristics of the dwellings and to the cultural level of their inhabitants. In the rural areas the low level of education and the larger size of dwellings favor the proliferation of the spiders and their persistence, causing a higher density of spiders per house than in urban houses. On the other hand, rural houses are scattered, limiting the migration of spiders to other houses. In contrast, migration from one house to another is common in urban communities and explains the higher percentage of houses infested. It is possible that the high proportion of immature spiders, and the sex ratio observed in adults, may be due to the methods employed in collection. Despite the high frequency and density of spiders in houses, the retiring nature of L. laeta reduces the hazard of poisoning in houses infested by these spiders.