The high variability in reproductive performance of North Atlantic right whales, Eubalaena glacialis, compared to southern right whales, Eubalaena australis, may reflect differences in lipid reserves. Amplitude-mode ultrasound was used to measure the thickness of right whale integument (epidermis and blubber; herein referred to as blubber thickness) in E. glacialis in the Bay of Fundy, Canada for 5 summer seasons and in E. australis off the South African coast for 2 austral winter seasons. E. glacialis had significantly thinner blubber layers (mean ± 1 SD = 12.23 ± 2.16 cm, n=172) than E. australis (16.13 ± 3.88 cm, n=117), suggesting differing levels of nutrition between the 2 species. Blubber was thickest in females measured 3 to 6 months prior to the start of pregnancy (E. glacialis); thinner during lactation (E. glacialis, E. australis) and then thicker with time after weaning (E. glacialis). These results suggest that lipids in blubber are used as energetic support for reproduction in female right whales. Blubber thickness increased in calves during suckling (E. glacialis, E. australis) but subsequently decreased after weaning (E. glacialis). Juvenile and adult male E. glacialis blubber thicknesses were compared among years of differing prey (Calanus finmarchicus) abundances (abundance data from Pershing et al. ): during a year of low prey abundance they had significantly thinner blubber than during years of greater prey abundance. Taken together, these results suggest that blubber thickness is indicative of right whale energy balance and that the marked fluctuations in North Atlantic right whale reproduction have a nutritional component.