In isolated head preparations of the freshwater crayfish Orconectes limosus 268 local and projecting interneurons with branches in the tritocerebrum have been penetrated with glass microelectrodes and characterized for their sensory inputs. Using 3 criteria (sensory modality, site of receptors, response type of interneurons), the interneurons found were divided into 16 classes. The interneurons were either unimodal mechanoreceptive (89%) or bimodal (9% responding to mechanical and chemical stimuli, 2% responding to mechanical and visual stimuli). No trimodal interneurons were found. Within each modality the neurons received mostly bilateral input (70% of all interneurons responding to antennal stimulation, 84% of all chemosensitive interneurons). If the input was lateralized it was more often ipsilateral. The types of interneuronal responses evoked by sensory stimulation were: neurons that were exclusively excited (84%), those that were exclusively inhibited (10%), those that were excited or inhibited depending on the modality or laterality of the stimulus (6%), those showing long lasting excitatory aftereffects (3%), and those showing excitation or inhibition upon identical stimulation depending on the state of the neurons while being stimulated (1%). Interneurons that responded to mechanical antennal stimulation responded best either to low (10 Hz) or to high (100 Hz) stimulus frequencies. Six neurons responded best to a certain phase relationship between the movements of both antennal flagellae.