Neurospora crassa is a potential expression system for evaluating fatty-acid-modifying genes from plants producing uncommon fatty acids. One such gene encodes the hydroxylase that converts oleate to ricinoleate, a fatty acid with important industrial uses. TO develop this expression system, it is critical to evaluate the metabolism and physiological effects of the expected novel fatty acid(s). We therefore examined effects of ricinoleate on lipid biosynthesis and growth of N. crassa. Ricinoleate inhibited growth and reduced levels of phospholipids and of 2-hydroxy fatty acids in glycolipids, but led to increased lipid accumulation on a mass basis. To evaluate incorporation and metabolism of ricinoleate, we followed the fate of 14 mu M-3 mM [1-C-14]ricinoleate. The fate of the [C-14]ricinoleate was concentration-dependent. At higher concentrations, ricinoleate was principally incorporated into triacylglycerols. At lower concentrations, ricinoleate was principally metabolized to other compounds. Thus, N. crassa transformants expressing the hydroxylase gene can be detected if the level of hydroxylase expression allows both growth and ricinoleate accumulation.