1. The maximum catalytic activities of glucose 6-phosphatase were measured in a large number of muscles from vertebrates and invertebrates. The activities range from less than 0.1 to 8.0 mumol/min per g fresh wt. at 30 degrees C: the highest activity, observed in the flight muscle of the wasp (Vespa vulgaris), is similar to that in rat liver. The hydrolytic activity was shown to be specific towards glucose 6-phosphate. 2. The pH optimum was 6.8 and the Km was approx. 0.6 mM (flight muscle of a moth). 3. Almost all of the glucose 6-phosphatase activity from extracts of the flight muscle of a moth and the pectoral muscle of a pigeon were recovered in the cytosolic fraction (i.e. 150,000 g supernatant). 4. During development of the locust (Schistocerca gregaria), the activity of the phosphatase in the flight muscle increased during the first 3 days after the final moult. 5. The activity of glucose 6-phosphatase from insect and avian muscle was separated from that of non-specific phosphatase on a Bio-Gel P-100 column. 6. For the activities from 63 muscles, there was a strong positive correlation between those of glucose 6-phosphatase and hexokinase, but no correlation between the activities of glucose 6-phosphatase and fructose bisphosphatase. It is suggested that the role of glucose 6-phosphate in muscle is either to produce glucose from glucose 6-phosphate derived from glycogen or to provide the enzymic basis for a substrate ("futile") cycle between glucose and glucose 6-phosphatase in muscle to improve the sensitivity of the mechanism that regulates the rate of glucose phosphorylation.