Ascaris suum L3 larvae isolated from rabbit lungs undergo the third ecdysis to L4 larvae after 3 days in culture under a gas phase of 85% N2/10% CO2/5% O2. The L3 larvae contain substantial malic enzyme activity and are capable of producing small amounts of the reduced organic acids characteristic of the fermentative pathways which operate in the adult. However, only a small portion of the total carbon utilized is accounted for by these reduced acids and their motility is cyanide-sensitive, suggesting that their energy-generating pathways are predominantly aerobic. In contrast, after ecdysis, the L4 larvae begin to utilize glucose at a greater rate and the proportion of total carbon utilized which is accounted for as propionate, 2-methylbutyrate and 2-methylvalerate also increases. In addition, motility becomes increasingly cyanide-insensitive, suggesting that these L4 larvae are able to utilize the anaerobic energy-generating pathways of the adult. Surprisingly, on day 10 in culture, these L4 larvae, although capable of producing reduced volatile acids, still retain substantial cyanide-sensitive cytochrome oxidase activity.