The effects of eyestalk ablation, cheliped amputation, and rearing conditions on molt increment and scute density were studied in larvae of the lobsterHomarus americanus. Eyestalk-ablated larvae had higher percentage increases in carapace length (ZCL) but lower scute densities (SDs) in the molts to third and fourth stage. Cheliped amputation decreased ZCL but not SD. Third stage larvae raised in a circulating seawater system and fed a diet of adult brine shrimp resulted in higher ZCLs but normal SDs compared with those raised in a static system fed a diet of brine shrimp nauplii. We conclude that the growth-promoting effect of eyestalk ablation was mainly a result of the increased surface area of epicuticle produced by each epidermal cell and that the postmolt size of intact larvae was predetermined mainly by the number of epidermal cells present before molt.