Teratological variations of Gomphonema parvulum Kützing were observed in a paddy drainage channel with effluent of domestic sewage. More than 40 % of the valves of G. parvulum were abnormal in winter. Abnormal frustules, including interrupted patterns of striations, irregularly directed striae and abnormal stigma, were of “Janus” type. The frequency of teratological forms was size-dependent. As valves decreased in length, the proportion of valves with abnormal striation increased. The senile change characterized by size reduction after rapid reproduction under high nutrient loading condition may cause those teratological variations.