The flagellate Euglena gracilis shows pronounced responses to light. The cells show positive photo-taxis (towards weak light) and negative phototaxis (towards high light) as well as pronounced step-up and step-down photophobic responses. The main photoreceptor of Euglena gracilis is the paraxonemal body (PAB), which is located in close vicinity to the trailing flagellum. The influence of inhibitors on phototactic orientation was tested. It was found that high concentrations of caffeine, a known phosphodiesterase inhibitor induce a sign change from negative to positive phototaxis at high fluence rates. The results are discussed with respect to results of Iseki et al. (Nature 415, 2002) and subsequent publications in this field, which demonstrate that the PAB shows a pronounced blue-light-activated adenylyl cyclase activity.