The molybdenum hydroxylases are a ubiquitous class of enzymes which contain molybdenum in association with a low molecular weight cofactor. Genetic evidence suggests that the Drosophila loci, ma--1, cin and lxd are concerned with this cofactor because mutants for any one of these loci simultaneously interrupt activity for two molybdenum hydroxylases, XDH and A0. A third enzyme activity, P0, is also absent in each of the three mutants but evidence classifying P0 as a molybdoenzyme has been lacking. This study utilizes the known tungsten sensitivity of molybdoenzymes to demonstrate directly that pyridoxal oxidase is also molybdoenzyme. The low molecular weight molybdenum cofactor is found to be severely reduced in extracts of the 1xd and cin mutants but ma--1 mutants have high levels of cofactor. A partially purified preparation of XDH crossreacting material from ma--1 was also shown to contain the molybdenum cofactor. These results, considered with data from other workers are taken to indicate that the functions of all three of the loci examined could be concerned with some aspect of cofactor biosynthesis.