Prior to reproductive maturity male and female worm pipefish Nerophis lumbriciformis are not significantly different in standard length (LS), eviscerated mass or condition indices. Subsequently, however, mature females achieve significantly (P < 0·001) greater values of LS and mass. For egg-bearing males, there was no marked difference in the mass of embryos with increasing development (P > 0·1); similarly there was no significant divergence in condition indices calculated for males carrying embryos of increasing development. Post-brooding males, however, had significantly lower values for the hepato-somatic index than either egg-bearing or non-mated male worm pipefish (P < 0·001). This study suggests that male worm pipefish invest a significant proportion of their energy budget into the development and maintenance of the brooding patch, to the detriment of increased length or mass.