In this study, the process of egg sac construction and the factors that determine fecundity in the spider Loxosceles intermedia were analyzed by comparing lab-reared females that had mated only once (n = 180 ovipositions) and females with unknown reproductive histories (n = 76 ovipositions). Among females known to have mated only once (n = 84), the number of viable eggs correlated positively with the duration of mating and with the age of the female at the time of fertilization and decreased significantly with successive ovipositions. In females with unknown (n = 36) reproductive histories, up to three fertile egg sacs were obtained from the same female with a third oviposition being observed only once. Oviposition was more frequent among larger females than smaller females. Among the reproductive variables evaluated, there were correlations between the number of eggs and the weight of the female spiders. More fertile eggs were laid by females with unknown reproductive histories than by females that mated only once. The existence of more stable environmental conditions, abundant food, and multiple fertilizations are probable factors which favor greater fertility of L. intermedia in urban Curitiba, located in southern Brazil, and can partly explain the success of this species in occupying this ecological niche.