1. The detectability of sinusoidal gratings comprised of either one or many cycles was measured in veiling luminances the spatial frequencies of which were either narrow- or broad-band.2. In narrow-band noise, the single-cycle grating was detected with approximately 0.6 log units less contrast than the many-cycle grating. On the other hand when both broad-band and narrow-band noise were present, there was no measurable difference in the detectability of the two types of grating.3. The results are interpreted as supporting the hypothesis of Campbell & Robson (1968) that spatially varying luminance patterns are processed by mechanisms selectively sensitive to limited ranges of spatial frequencies.