The enzyme carbonic anhydrase (CA) is known to be salinity-sensitive, increasing 10 fold in gills of blue crabs transferred from high to low salinity. CA activity and ornithine decarboxylase (ODC) activity were measured in anterior (G3) and posterior (G7) gills of blue crabs transferred from 35 to 25 ppt, and from 25 to 15 ppt salinity. In addition, hemolymph osmolality and total ninhydrin positive substances (TNPS) were also measured as indicators of hemolymph osmotic regulation and cell volume regulation, respectively. CA increased slightly between 35 and 25 ppt, but there was a large cell volume readjustment that occurred soon after transfer and was complete by 24 hr. For transfer from 25 to 15 ppt the cell volume response was minimal, but there was a large increase in CA activity in G7, beginning at 24 hr post-transfer. ODC activity did not change significantly in either G3 or G7 for either transfer. Both cell volume regulation and transport-enzyme induction in low salinity appear to be independent of ODC activity.