The sex ratio in five populations of the sheetweb spider Pityohyphantes phrygianus in southwest Sweden was investigated in July and September, the spiders being juveniles and subadults, respectively. In July, no heterogeneity between the five populations was found and the pooled samples revealed a sex ratio of 36% males and 64% females. A significant heterogeneity in sex ratio was observed in the September samples; the proportion of males varied between 23% and 40%. In several cases, male mean weight differed significantly between the five populations, comparing the July and September samples separately. This suggests that the prey availability varied considerably between the sites. In September, dead spiders were collected in two sites. This mortality was probably caused by starvation and the mortality rates differed significantly between the two sites. Indirect evidence suggested that the local sex ratio was affected by the prey availability, males being at a relative disadvantage in poor sites. Local environmental conditions may govern the adult sex ratio in this spider by causing differential mortality and/or migration.