Podophthalmus vigil (Fabricius) and Portunus sanguinolentus (Herbst) percept tactile, osmotic, and pH stimuli with their antennulae. The receptors are located in the plume-like flagellum. Diluted seawater, when applied to this flagellum, causes an increase in electroactivity in the antennula; the lower the salinity the greater the activity. Podophthalmus seems to be more sensitive to dilution than Portunus. Concentrated seawater (125%) does not elicit a response in either. Changes (pH) of the seawater, both to the alkaline, and to the acid side, evoke electrical responses. Seawater acidified with CO2 has the same effect as seawater acidified to the same pH with HC1. Tactile stimulation of legs, antennae, and mouth parts causes a feedback into the antennulae. The meaning of the sensoric component of this feedback remains obscure.