The interaction of isovaleraldehyde, an attractant, with zoospores of the fungus Phytophthora palmivora was investigated by using binding techniques. The amount of isovaleraldehyde bound diminished with time, an effect that may be related to sensory adaptation. In addition to non-specific binding, specific and saturable isovaleraldehyde binding was demonstrated. Cooperativity occurred at intermediate but not at low and high ligand concentrations. [3H]isovaleraldehyde was displaced by unlabelled isovaleraldehyde and by a number of chemically related ligands known to be chemotactic agents.l Their dissociation constants as calculated from the displacement experiments paralleled their potency as attractants and it is suggested that these attractants act through the same receptor. Some other chemotactic agents did not displace [3H]isovaleraldehyde, and these presumably act through one or more other receptors.