During hurricane “Hazel”, on October 15, 1954, the Holland Marsh, Bradford, Ontario, was flooded with water to an average depth of about four feet. Some sections were covered by more than 10 feet of water and remained flooded for two to three weeks. Data from population studies on the carrot rust fly, Psila rosae (F.), conducted in the area by Miss J. B. Williams of the Ottawa laboratory from 1951 to 1953 and by the author in 1954 and 1955 provided an opportunity to determine whether the flood had affected the abundance of the insect.