Abstract
1. Several species, which are presumed to be aberrant form (flavism) of Prosimulium yezoense Shiraki, 1935, were collected together with normal individuals at the mountains on Nagano and Yamanasi Pref., Japan. 2. This species was named P. yezoense ab. alpium n. ab.. 3. P. yezoense is a typical mountain breeding species in Japan. The early stages are found in swift mountain streams, and the adults attack man and animal from March to July. 4. The aberrant form is distinguished in the following points from the normal form : (a.) Antenna is blackish brown, with basal two segments and basal half of the third segment is yellowish brown. The latter is black. (b.) Mesonotum, pleurae, scutellum and basal scale is yellow or yellowish brown, with the bright golden pubescence or hairs. The latter is black, with the bright golden pubescence or hairs. (c.) Abdomen is yellowish brown, and becomes blackish brown gradually to posterior end. The latter is blackish brown or black.
