The spiny rock lobster (Jasus frontalis) stock exploited in the Robinson Crusoe and Santa Clara islands of Juan Fernandez archipelago (33°40'S-80°00'W), is assessed and the exploitation state is analyzed. An analysis of cohort at size (ACT) model is adjusted to the fishery carried out in the 1996-97 season. The fishing mortality of the totally recruited groups (Fcr) are estimated in 0,63 for males and 0,81 for females, with a 50% of retention size of 94 to 96 for males and 96 for females. The lobsters mean biomass is estimated in 29 to 31 ton (46.663 and 50.355 individuals respectively) for males and in 39 to 42 ton (59.785 and 63.249 individuals respectively) for females. On the other hand, in the equilibrated yield analysis is estimated that fishing mortality Fcr derived of the ACT model, was inferior to the Fmax recommended value estimated in 0,86 for males and 1,15 for females, and superiors to the F0.1 mortality. The long term equilibrated yield analysis (CBA), suggests for a F0.1 exploitation strategy that the recommended landing value is equal to 10,4 and 14,6 ton (16.885 and 15.449 individuals) of males and females respectively, 10% inferior to current landings levels. The critical size of lobster for a value of M=0,18, was determined similar to 110 mm in males and 107 mm in females, while the calculated critical age for both sexes corresponds to the group of 9 year-old age. The lobster state situation indicates that the current exploitation levels are intense and with exploitation rates that would reduce the mean biomass in the long term. The biomass would reduce the yearly total levels to 50 ton (males + females), significantly inferior magnitude if an exploitation rate equivalent to F0.1 biological point will be implemented, which would reach a yearly population equivalent size of 70 ton.