A girl had generalized, rapidly progressive weakness beginning at age 4 weeks, and causing severe respiratory insufficiency and death at age 13 weeks. Histochemical and biochemical investigations of a muscle biopsy showed increased glycogen concentration and complete lack of phosphorylase activity. The enzyme protein appeared to be absent by immunodiffusion, and the metabolic block was documented by studies of anaerobic glycolysis in vitro. The biochemical basis for the unusual clinical picture is obscure, but muscle phosphorylase deficiency has to be considered in the differential diagnosis of the "floppy baby syndrome".