Myofibrils which lengthen by several per cent in the presence of ATP and magnesium ions were prepared by teasing indirect flight muscle of Drosophila in solutions containing ethylenediaminetetraacetate. A study was made of the hydrogen ion, magnesium ion, ATP, and potassium chloride concentrations with which this effect could be observed. The lack of elongation with pyrophosphate and several nucleoside triphosphates suggests that the lengthening is ATP specific. A relaxing factor system comparable to that described for rabbit muscle was not demonstrable, as elongated fibrils did not shorten with calcium ions, carnosine, or digitonin.