Abstract 1. 1. Gonadal indices (i.e. GSI = gonadal wt/ body wt × 100) commonly are used to quantify reproductive condition in fishes. These indices may be inappropriate with specimens of different sizes. however, for gonActal growth often is allometric. 2. 2. A new gonadal index (relative gonadtal index, RGI) was developed to quantify the reproductive condition of animals independent of body size. 3. 3. The RGI is based on the underlying model W = α i S βi , where W is gonActal weight, S is body size (less gonadal weight if body weight is used), and α, and β, are parameters to be estimated for gonadal developmental stage i. 4. 4. Assuming that a multiplicative lognormal error is appropriate, parameter estimates for α i , and β i were obtained by linear least squares regression for the log-transformed model ln( W) = β i - ln( S) + ln( α i ), where, in this form, β i is the slope and ln ( α i ) is the intercept. 5. 5. Only if estimates of β i , do not differ significantly among ovarian developmental stages, as in our case, can a pooled estimate of β be used to obtain the relative gonActal index, RGI = α i = W/ S β . 6. 6. Applicability of the RGI was tested using ovaries of three ecologically distinct fish species. The RGI was found to be more appropriate than the gonosomatic index for all three species.