Summary Heterochrony, which is a change in the developmental rate and timing, is one of the mechanisms proposed to explain alterations in meristic characters, such as the variation in the numbers of skeletal elements between individuals within a population. To verify the possible role of heterochrony in the evolutionary transformation of the cyprinid pharyngeal tooth formula, the development of the pharyngeal dentition in zebrafish (Danio rerio) was studied under the influence of deficiency of thyroid hormones at 28°C and standard zebrafish rearing conditions. Thiourea was used to decrease the level of thyroid hormones, a treatment known to cause retardation of several developmental processes, including skeletal development and tooth formation. Retarded development resulted in a high degree of variability as well as left – right asymmetry in the dentition. Supernumerary teeth developed; these were either extending a tooth row anteriorly or were added lateral to a tooth row, or both. An addition laterally resembled the start of formation of an accessory tooth row. Compared to younger individuals, older zebrafish showed a higher number of supernumerary teeth, often associated with a broadened attachment zone on the jaws. Both anteriorly and laterally positioned supernumerary teeth were regularly displayed. These results support the hypothesis that heterochrony can affect dental characters in cyprinids, as has been suggested in the development of the skull, caudal fin and the Weberian apparatus.