A new method was developed to estimate the gonado-somatic index in bivalves, taking into account that in most bivalves gonad and non reproductive tissues are fused. As examples, four tropical bivalve species (Donax dentifer, Cardita affinis, Pinctada imbricata and Pteria colymbus) from the Colombian Pacific and Caribbean were used. The fused gonad regions were replaced by geometric bodies, thus the gonad volumes were estimated with geometric equations (corresponding to geometric bodies) and linear measures obtained directly on frozen soft-body samples.The very good correlation between the gonado-somatic index and the gametogenetic cycle of the stage "ripe", leads to the conclusion that this method is a suitable tool to estimate gonad indices of bivalves with fused gonads.