Estimates of mean monthly and mean annual streamflow were made for 56 selected sites in the Musselshell River basin and 1 site outside the basin. The study area was divided into a Mountain Region and a Plains Region and the estimation methods were applied separately in the two regions. Four methods were developed to estimate mean monthly streamflow at ungaged sites. The first method was based on the regression relation between mean monthly streamflow and various basin and climatic characteristics. The standard errors ranged from 35 to 71% in the Mountain Region and from 98 to 157% in the Plains Region. The second method was based on the regression relations between mean monthly streamflow and activechannel width. The standard errors ranged from 38 to 81% in the Mountain Region and from 71 to 98 in the Plains Region. The third method was based on correlation of measured streamflow at ungaged sites with concurrent daily mean streamflow at nearby gaged sites. The standard errors ranged from 36 to 66 % in the Mountain Region and from 109 to 321% in the Plains Region. The fourth method, generally the most reliable, estimated mean monthly streamflows by weighing individual estimates in accordance with their variance and degree of independence. The standard error for this method when all three individual estimates were weighed ranged from 25 to 55% in the Mountain Region and from 71 to 97% in the Plains Region.