Size variables in puffins F. arctica (family Alcidae) from 2 Icelandic colonies, one in the North and one in the South, were examined. Generally, male puffins were larger than females. A marked size difference was shown between puffins from the 2 colonies, birds from North Iceland being larger. A comparison of size was made between Faeroese puffins (F. a. grabae) and puffins from South Iceland (F. a. arctica). Differences between birds from these 2 populations were smaller than those found between puffins from South and North Iceland. This contradicts conventional thinking that all Icelandic puffins belong to the same subspecies. Some discussion is devoted to nomenclature difficulties in F. arctica. For the 1st time bill features were proven indicative of age in puffins. Descriptions are given of puffins 2, 3, 4, 5 and 6 yr and older from the South Iceland colony.