This study aimed to provide information about the population structure and somatic index of Hypostomus cf. ancistroides collected from the Bonito river, located in the Ivaí river basin. The length-weight relationship was isometric for both the sexes. The length structure analysis showed that the larger individuals (from 18.1cm to 27.0cm in length) predominated, and the lowest abundances occurred at the size extremes (9.1-12cm and 27.1-30.0cm). The reproduction period occured between October and January for the females and between November and January for the males. The liver somatic index cannot be used as an indicator of the reproduction period in either of the sexes, due to no correlation between the liver somatic index and the gonad somatic index. The gonad weight exerted no influence on the monthly mean condition factor and the correlation between the condition factor and gonad somatic index was high. The condition factor could be an indicator of the reproduction period of this species.
Gonad somatic index; liver somatic index
