The spatangoid echinoids Abatus shackletoni and A. nimrodi were examined for reproductive activity in the early austral spring and summer of 1984 and 1985 in McMurdo Sound, Antarctica. Gonad indexes did not change over the sampling period for either species, nor did they differ between the sexes. The testes always contained spermalogcnic columns and numerous sperms; the ovaries always contained many different-sized oocytes. Over 60% of the females in all samples contained embryos in the brood chambers, and these were in different stages of development within individuals. These observations suggest that, unlike A. cordatus of the Kerguelen Islands, the two common species of Abatus in McMurdo Sound reproduce throughout most or all of the year. However, like A. cordatus, mean number of brooded embryos was about 30 in both species (maximum number of 102 embryos in A. shackletoni and 129 in A. nimrodi, versus 109 in A. cordatus). There were substantial differences between the two species in McMurdo Sound, which were spatially segregated; A. nimrodi occurred in the oligotrophic western part of the Sound while A. shackletoni was found in the seasonally productive eastern part. Individuals of A. nimrodi were about twice the size of those A. shackletoni, their gonad indexes 3-5 times higher, and the energy content (per gm dry weight) in their body tissues (both somatic and reproductive) about 30% higher. Reproductive effort, both absolute and relative, was thus considerably higher in A. nimrodi, which also produced larger eggs, 1.97 mm in mean diameter versus 1.28 mm in A. shackletoni. The larger embryos of A. nimrodi developed into larger juveniles before release from the brood chambers. Populations of the two species may experience different levels of juvenile and adult mortality, which may be reflected by different reproductive tactics. © 1990 Balaban.