• allometry;
  • gonad;
  • sperm competition;
  • testes

The allometric relationship between body mass and gonad mass in males of 23 fish species from 11 families was examined. There was no evidence of a single allometry for all fishes. A cross species analysis suggested a scaling coefficient of 1·04, which was significantly different from a previous study that reported a scaling coefficient of 0·904. A within species analysis generated scaling coefficients from 0·68 to 3·90. Furthermore, for those species characterized by alternative mating tactics, there was no correlation between the scaling coefficients of bourgeois-type males and parasitic-type males. These results are discussed in the context of the gonado-somatic index and its use in testing sperm competition theory.