T HE Montana grayling (Thymallus montanus), like other species of the Thymallidae, breeds in the spring of the year. Milner (1874) gave April as the time of spawning for the Michigan species (T. tricolor), while Whitaker (1886) included March and April (and perhaps even earlier). Norris (1883) considered it to be a late spring spawner, referring to April and early May as the breeding period. According to von Siebold (1863: 270) the European grayling (T. thymallus) begins to spawn in March and may continue into April. The actual period of spawning for the Montana grayling has been found to vary greatly between different years and between different localities in the same year. Whereas the controlling factors which initiate the breeding activity of this species have not been carefully studied, it is safe to say that the temperature of the water has great influence. In 1936 on Lower Odell Creek, tributary to Madison River, Montana, the first grayling came into the U. S. Bureau of Fisheries traps on March 15. The peak of the run was attained April 16 and the last individual observed appeared April 24. The same year a few spawners were taken in the Meadow Creek traps (a few miles downstream from the Odell Creek station) between May 1 and May 15. The spawning period seems to be much more concentrated in the artificially stocked lakes of the region. Experience has shown that grayling often run in advance of the rainbow and cutthroat trout and that the males frequently appear in advance of the females. At Grebe Lake, Yellowstone National Park, the run usually begins the day after the ice goes off. During the 5 years previous to 1937 the spawning period occurred at Grebe Lake between May 15 and June 20; at Rogers Lake, Flathead County, Montana, between May 1 and June 1; and at Georgetown Lake, Granite County, Montana, between May 10 and June 5. In these lakes the grayling run is much more concentrated than the trout run, usually lasting not over a week and often being confined to 2 or 3 days. Henshall (1907) reported that the Montana grayling will travel long distances to find suitable spawning grounds. This is in contrast to published records for the Michigan species, which according to Norris (1883) does not migrate as do the trout. Bissel (1893) said of the Michigan grayling, ... they use the channel of the main stream not seeking the brooklets and shoals as trout usually do. Von Siebold (1893: 270) observed that the European grayling did not migrate during the breeding period. Certain observers, however, describe this species as running with the trout in English streams. There is no doubt that the Montana grayling migrates during the spawning period. Those in lakes move out into whatever streams are available and those in streams go varying distances depending upon the stream in question. Henshall