Abstract Thresholds were measured for a tiny, brief, violet flash on a long wavelength, B cone-isolating background in foveal locations spaced only 4 or 5′ of arc apart. Large spatial variations in B cone sensitivity were found just beyond the foveal tritanopic area even though thresholds for the same wavelength test flash hardly varied at all across these same retinal locations when the flash was detected by G cones. The relative constancy of G cone threshold suggests that these spatial variations are intrinsic to the blue-sensitive mechanism and cannot be explained by prereceptoral filtering. The spatial variations in B cone sensitivity are consistent with physiological evidence that B cones are scarce in the retina. In one observer, it was possible to discern discrete peaks in sensitivity spaced roughly 10′ of arc apart. A model is described which takes optical spread and eye movements into account to show that these peaks may represent individual B cones (or clumps of B cones).