Abstract. Abstract: Aim To investigate reproductive characteristics in a small population of Psalidodon aff. fasciatus living in isolation in an environment with a unique geological configuration, a doline-shaped depression (known as Furna 2), with a mean diameter of 80 m and water depth of 50 m, a natural heritage of the Vila Velha State Park (VVSP). Methods The fish were collected between February 2019 and January 2020 using nets and pots; standard length and total weight were measured. The gonads were also removed for histological analysis. Estimates were made for the weight-length ratio, the relative condition factor, the sex ratio, and the size at first maturity (L50 and L95) for males and females. Results The five gonadal stages that were characterized (resting, early mature, mature, partially empty, and totally empty) were consistent with what was expected for the species. The results obtained here indicate that these lambaris have an opportunistic (r) life strategy. Compared to previous studies, we discovered that fish of this population start maturing at smaller sizes. Indicating negative allometric growth, the length-weight ratio was WT = 0.0002 * LT2.3039, and the condition index (Kn = 0.95) showed no significant gender differences. When compared to other non-isolated populations of the same species, the population's sex ratio showed an unexpectedly significant male predominance (1M: 0.6 F). Conclusions Our results show an imbalance in the sex ratio, contrary to what would be expected for non-isolated populations. This imbalance may be related to the complete isolation of this environment, suggesting that females may be affected by some evolutionary force, such as inbreeding. However, other factors cannot be ruled out, as the underlying genetic mechanisms remain unknown, and only future investigations can reveal a more complete understanding of reproduction in isolated populations.