The biophysical properties of the surface lipid layer (the epicuticle) of living parasitic nematodes (Trichinella spiralis and Toxocara canis) were examined using fluorescent lipid analogues. A variety of such probes were screened, and only 5-N-(octadecanoyl)-aminofluorescein was found to insert into the outer lipid layer. Fluorescence quenching experiments showed that this probe was confined to the surface, and the rate of its lateral diffusion was then measured by Fluorescence Recovery After Photobleaching. This showed that the probe was not free to diffuse within the plane of the epicuticle. This structure is, therefore, extraordinary in its selectivity to lipid probes, and in the restricted lateral mobility of inserted lipid components.