An investigation was made of the food, spawning runs, age and growth, and distribution of the longnose sucker Catostomus catostomus in Yellowstone Lake during the summers of 1951 and 1952. This fish was probably accidentally introduced into Yellowstone Lake by bait fishermen in the early thirties. As many as 2,000 specimens were observed with the annual cutthroat-trout runs in Pelican Creek during the early forties. A total of 1,150 was counted there in 1951. The food of suckers captured in the mouths of tributary streams showed an incidence of 69 percent algae, 40 percent higher aquatic plants, and numerous aquatic insects, particularly Diptera. During the spawning run the largest number of suckers entered the traps on the days when water temperatures were highest. Females were significantly larger than males. The average time spawning fish stayed in Pelican Creek was 19 days for females and 17 days for males. The average loss of weight for spawning females was 4.7 ounces and for males 1.8 ounces. A sample of sucker fry captured on July 9 averaged 13 millimeters in total length. Young taken October 11 averaged 30.5 millimeters. The calculated total lengths for each year of life based on 525 specimens were as follows: first year–2.0 inches; second–4.8; third–8.5; fourth–11.7; fifth–13.6; sixth–15.8; seventh–17.4; eighth–18.1. Females grew more rapidly than the males and reached a larger size. Tagging experiments showed that suckers have a rather restricted range and are frequently taken by pelicans.