The cucumber odor emitted by the European smeltOsmerus eperlanusand Canadian surf smeltHypomesus pretiosus(both Osmeridae) and by the New Zealand common smeltRetropinna retropinnaand Stokells smeltStokellia anisodon(Retropinnidae) is shown to be caused by trans-2 -cis-6-nonadienal, as has previously been shown for the Australian graylingPrototroctes maraena(Prototroctidae). These fish are all members of the Osmeroidei. However some other groups (Argentinidae, Chlorophthalmidae, Pomatomidae) are also reported to emit the odor and it is not presently possible to determine whether the odor is a commonly derived homologous character within the Osmeroidei and is convergently present also in other families, or whether it is a primitive character in the Osmeroidei and other related groups. The function of the chemical, which occurs in the skin, is unknown, and clarification depends on understanding metabolic pathways involved.