A technique for age determination of young Montana blue grouse (Dendragapus obscurus pallidus) based on measurements of actively growing juvenal and postjuvenal primary feathers of 17 pen-raised birds is described. When tested on 87 approximate known-age wild birds to 16 weeks, 95 percent of 116 determinations were within 1 week of the expected age. Supplemental characters for aging juveniles in the field are presented. Sound game management requires accurate information about annual production. Data on upland game bird productivity are gathered in various ways, one of the easiest and best being hunting-season wing collections. However, production data are needed from an earlier period in order to set opening dates, lengths, and bag limits. Early studies of Montana blue grouse (Mussehl 1960) indicated the need for an accurate method of estimating the age of juveniles captured during field studies. A method was also sought which could be applied to birds observed but not captured. Time of molt of individual primaries, and length of growing primaries are both important indicators of age in juveniles of most upland birds and have been widely used as a means of determining the age of juveniles as well as for dating breeding season events. Previous studies of numerous upland bird species have described the juvenile primary molt in various ways (Taber 1969:326-332). Bendell (1955:354-355) and Boag (1958: 8-10) described the general molt progression of juvenile blue grouse, while Mussehl (1960:63) reported similarities between the primary development of juvenile blue grouse and that given for ruffed grouse 1 A contribution from Federal Aid in Wildlife Restoration, Montana Project No. W-91-R. (Bonasa umbellus) by Bump et al. (1947). More recently, Smith and Buss (1963) and Zwickel and Lance (1966) provided techniques for age determination of juvenile blue grouse by the stage of growth of juvenal and postjuvenal primaries. Boag (1965: 107, Table 1) used the graphs of Smith and Buss (1963:568-569) to estimate the age of two wild juvenile blue grouse in Alberta and reported that the method appeared reliable (Boag 1965:108). Zwickel and Lance (1966:715) tested their technique on 73 wild chicks, finding 35 of 36 determinations to be accurate to within 2 days at under 26 days of age. Their accuracy decreased after this time but 85 and 95 percent of their determinations were within 3 and 5 days of the expected age, respectively. They noted a decrease in accuracy with increasing age, but were unable to test their technique on wild birds over approximately 66 days of age. Zwickel and Lance (1966) compared their data with those of Smith and Buss (1963) and found the latter's birds to be slower in molt and primary development. Zwickel and Lance (1966) suggested that either a difference between races, different rearing techniques, or a slowly developing brood of Smith and Buss may have contributed to the differences between their studies. Our study provides an additional