No eclosions were obtained in D. melanogaster from a medium containing >0.25% formaldehyde. Eggs 1-4 hrs. old were collected on agar and allowed to hatch and develop for an additional 48 hrs. The larvae were transferred to formaldehyde-containing media. Eclosions in untreated controls were almost 100%. The Muller-5 method of detecting lethals in the X chromosome was used. The newly hatched treated [male][male] were crossed to Muller-5 [female][female], and the daughters were crossed to Muller-5 d10" A lethal mutation was indicated when there were >4 [male][male] in the culture, none of which had the wild type eye. There was a very significant difference in the % of lethals obtained by the use of formaldehyde and that obtained in controls. There was no correlation between the concn. of formaldehyde and the induced rate of mutation.