The firefly squid, Watasenia scintillans, is the only animal known to possess a visual pigment in which the chromophore is 4-hydroxyretinal. This paper describes the absorbance spectrum and some properties of a synthetic "A4" visual pigment generated from bovine opsin and 4-hydroxyretinal. The absorbance spectrum of this pigment is compared with (a) bovine rhodopsin and (b) a rhodopsin template with the same λmax as the synthetic visual pigment. The A4 pigment is shown to have an absorbance spectrum that is almost identical to that of a rhodopsin template. It is also shown that the photosensitivity and thermal stability of the A4 pigment, dispersed in detergent micelles, is essentially similar to that of rhodopsin.