Morphological deformities in Prodiamesa olivacea larvae from an urban contaminated site in Santiago de Compostela (NW Spain) are documented and illustrated. Anomalies in this species were compared to those present in Chironomus riparius larvae collected at the same site. Deformity frequencies in months with N ≥ 30 larvae ranged from a low of 2.3% to a maximum of 9.1% for Prodiamesa olivacea, while those for Chironomus riparius ranged from 10% to 37.5%. Due to the increasing interest in using deformities for biomonitoring studies, the ‘Toxic Score' for Chironomus larvae is utilised with Prodiamesa olivacea. Scoring system results for both species show a significant correlation for data of months with N ≥ 30 larvae of each species. Hence, deformities in Prodiamesa olivacea would be useful for biomonitoring purposes during periods of the year when Prodiamesa is much more abundant than Chironomus, or in other situations when Chironomus is either absent or present in low numbers.