A new index was used to describe and quantify claw asymmetry for American lobsters, Homarus americanus. Length : width (L:W) ratios were calculated for each claw, and a measure of claw asymmetry (C_a) was computed as the percent reduction in the claw with the smaller L:W ratio. Fourth stage (first benthic stage) lobsters had relatively symmetrical claws, with L:W ratios of 4.3 in the cutter and 4.1 in the crusher claw, and a corresponding C_a value of 5.4%. Hatchery animals that could not differentially exercise one claw maintained a low average C_a value (2.5%). In adult animals, the L:W ratio decreased to a minimum of 2.7 for cutter claws, and 2.0 for crusher claws, giving a C_a value of 25.9%. This method was then used to assess claw development in animals exposed to one of four different natural substrates (cobble, shell, sand, and plant) in a hatchery setting. The development of claw asymmetry is known to be a function of increased exercise in one claw, and substrates allowing for more exercise should exhibit faster, greater asymmetry. It was observed that the overall morphology of claws changes greatly between the sixth and seventh stage. Although no treatment differences were observed at the sixth stage, by the seventh stage, animals subjected to shell substrate yielded significantly more asymmetrical claws. Thus, where lobsters settle can have a dramatic impact on the rate of development of claw asymmetry with potential fitness consequences.