Annual fluetuation of densities and population strueture of Armases angustipes (Dana, 1852) from a rocky shore at Farol Island, Matinhos, Paraná, Brazil (25º51'S and 48º32'W) were studied. The population strueture was treated in the following aspects: sexual proportion, dial intensity of activities, size composition and its fluetuation, and general condition of the crabs. The collections were made monthly, from January to December 1991. The samples were obtained handly, in the morning and at night, at the supratidal zone during low tides. There were caught 1,139 crabs, among which 34 were juveniles, 544 males, 555 females and 4 ovigerous females. Population density was highest in April (19.56 ind.m"² ) and in October (23.86 ind.m"² ). The sexual proportion was 1:1 in eleven months. Although the male crabs were more abundant than the females in May, there was not any statistical significam difference between the sexes. The activity of the population is intenser at night than at morning for both sexes. The recruitment of juveniles oceurs mainly in March, April and July. The condition factor in females is related to the reproduetive period. The population can migrate to the terrestrial habitat near the supratidal zone.

The relative growth of the male chelipod and female abdomen of Armases angustipes (Dana, 1852) was studied in order to determine the carapace size of puberty. Also, the age of juvenile crabs in laboratory was established. A total of 537 males and 557 females were analysed and the transition point between mature and immature stages was determined for both sexes. Males attain maturity at 1.00 cm carapace width and females at 1.08 cm. The relative growth of female abdomen was more remarkable than that of the male chelipod, although both are positive allometric. It suggests a greater importance of a large abdomen in females than a strong chelipod in males. Juvenile crabs reared from megalopa stage has five developmental stages before reaching sexual differentiation. This occurs in the sixth juvenile stage, when the intermolt stages become longer. The potential equation between carapace width and accumulated duration of juvenile stages is described as CW = 0,047D0,508.