The mas-1 gene of Drosophila melanogaster encodes Golgi mannosidase I (MAS-1), and flies homozygous for small deletions of the gene are viable. They show but few abnormalities and those have a low penetrance [Kerscher, S., Albert, S., Wucherpfennig, D., Heisenberg, M. & Schneuwly, S. (1995) Dev. Biol. 168, 613-626]. Here we sequence the N-linked oligosaccharides associated with a reporter protein, and with membrane proteins prepared from wild type and MAS-1 null organisms. The results show that the null organisms synthesise the same range of oligosaccharides as wild type, albeit with different ratios. There is an accumulation of the Man8GlcNAc2 which is one of the substrates for the MAS-1 enzyme. This supports the suggestion of Kerscher et al. that the lack of severe phenotypic disturbances in the null organisms is due to the presence of an alternative pathway(s), but it also shows that this alternative pathway(s) does not entirely compensate for the normal pathway.