From 1983-1996, the iris colour of 45 to 65 captive Humboldt Penguins Spheniscus humboldti at Emmen Zoo, The Netherlands, was monitored monthly. As chicks and juveniles, Humboldt Penguins at first have dark (grey) indes, which become pale after a while. In most adults the pale indes eventually become red, but some individuals continue having pale irides. Males attain red irides at an earlier age than do females. Two very old males were found to have black irides. In winters on average a slightly redder iris than in the following summer was observed. Iris colour between and within penguin species is darker at higher latitudes. Possible explanations for this are discussed.