The kairomone released by Chaoborus (phantom-midge) larvae induces neck protuberances (neckkeel with neckteeth) in the dorsal anterior margin of the head of Daphnia that are part of the defense response of this cladoceran against predation pressure. In aquatic ecosystems, kairomones are exposed to different environmental factors that may affect their fate. Among them, solar ultraviolet radiation (UVR) is known to alter chemical moieties and degrade organic substances. In this study, we tested the hypothesis that UVR alters kairomones reducing their efficiency as infochemicals. A kairomone extract from Chaoborus obscuripes pre-exposed to UVR for 5 or 10 h was assessed for the induction of neck protuberances in Daphnia pulex and compared with a control (i.e. with the kairomone unexposed to UVR). The results indicated that after 5 h exposure to UVR, the kairomone was photoaltered and the formation of neck protuberances strongly reduced (number of neckteeth by 31% and height and with of neckkeel by 55 and 43%, respectively). Exposure of the kairomone for additional 5 h did not result in further reduction of neck protuberances. This is the first report indicating that UVR has the potential to alter infochemicals in the aquatic realm and thus, to indirectly affect predator–prey interactions.