Most non-primate mammals have two types of cone: short-wavelength sensitive (S) and middle-to-long-wavelength sensitive (M/L) cones. In two species of African giant rats, Cricetomys gambianus and C. emini, and in two species of earless seals, Phoca hispida and P. vitulina, the retinal cone types and cone distributions were assessed with antibodies specific for the M/L-cone opsin and the S-cone opsin, respectively. All four species were found to completely lack S-cones, while M/L-cones were present in low densities. M/L-cone densities, rod densities and cone/rod ratios were determined across the retina. Cone proportions are about 0.3–0.5% in C. gambianus, 0.5–0.8% in C. emini, and 1.5–1.8% in P. hispida. An absence of S-cones has previously been reported in a few nocturnal mammals. As earless seals are visually active during night and day, we conclude that an absence of S-cones is not exclusively associated with nocturnality. The functional and comparative aspects are discussed.