Egg capsules of the gastropod Acanthina monodon were maintained during the entire period of encapsulated development at three temperatures (10, 15, 20 degrees C) and two pCO2 levels (400, 1200 mu atm). Embryos per capsule, size at hatching, time to hatching, embryonic metabolic rates, and the resistance of juveniles to shell breakage were quantified. No embryos maintained at 20 degrees C developed to hatching. The combination of temperature and pCO2 levels had synergistic effects on hatching time and developmental success, antagonistic effects on number of hatchlings per capsule, resistance to juvenile shell cracking and metabolism, and additive effect on hatching size. Juveniles hatched significantly sooner at 15 degrees C, independent of the pCO2 level that they had been exposed to, while individuals hatched at significantly smaller sizes if they had been held under 15 degrees C/1200 mu atm rather than at 10 degrees C/low pCO2. Embryos held at the higher pCO2 had a significantly greater percentage of abnor-malities. For capsules maintained at low pCO2 and 15 degrees C, emerging juveniles had less resistance to shell breakage. Embryonic metabolism was significantly higher at 15 degrees C than at 10 degrees C, independent of pCO2 level. The lower metabolism occurred in embryos maintained at the higher pCO2 level. Thus, in this study, temperature was the factor that had the greatest effect on the encapsulated development of A. monodon, increasing the metabolism of the embryos and consequently accelerating development, which was expressed in a shorter intracapsular development time, but with smaller individuals at hatching and a lower resistance of their shells to breakage. On the other hand, the high pCO2 level suppressed metabolism, prolonged intracapsular development, and promoted more incomplete development of the embryos. However, the combination of the two factors can mitigate-to some extent-the adverse effects of both incomplete development and lower resistance to shell breakage.