Nucleoside auxotroph; less than 5% of the expected number of homozygous progeny (from a cross of homozygous females to heterozygous males) are found on nucleoside-free medium. Those homozygous flies which are found develop slowly. Many more homozygous progeny are produced in the reciprocal cross of heterozygous females to homozygous males on restrictive medium and developmental delay is reduced. Progenies of matings between heterozygous flies contain substantial numbers of homozygous offspring, whereas crosses between homozygous flies produce exceedingly few offspring on restrictive medium. nuc1¹ responds to dietary ribonucleosides (uridine, cytidine, adenosine, guanosine and inosine); uridine and cytidine supplement well at 5x10^-3M, whereas adenosine, guanosine and inosine are not as satisfactory. nuc1¹ responds less well to bases or pyrimidine precursors.