The main constituent of cod otoliths, calcium carbonate, has been shown to be present in the form of aragonite. We have investigated the nature of the aberrant forms which occasionally occur, since it was thought that this knowledge might be of use in studying the factors affecting the structure and growth of normal otoliths. There appear to be 2 main aberrant forms, which we have designated Type 1 and Type 2. In the Type 1 modification the nacreous smooth-ribbed structure of the normal cod otolith is completely lost, being replaced by a translucent randomly-crystalline structure which does, however, retain the general overall shape of the normal otolith. The Type 2 modification partly retains the structure of the normal otolith, but a portion is covered by markedly crystalline accretions which may cause considerable distortion of the normal shape. X-ray powder analyses of whole Type 1 otoliths and the crystalline accretions from Type 2 otoliths showed that Type 1 otoliths contain a mixture of calcite and vaterite while the accretions from Type 2 contain calcite only. The occurrence of vaterite in the Type 1 aberration is interesting since this modification of calcium carbonate only appears to have been found once in plant or animal sources before namely in small amounts in human gallstones.