Three mutant strains of the phytoflagellate Euglena gracilis Z have been characterized in order to analyze the signal perception and signal transduction pathways involved in photo- and gravitaxis. Using the fluorescence of the chromophoric groups believed to be involved in photoperception (flavins and pterins) a method was developed for an in situ and in vivo detection of the paraxonemal body, the proposed location of the photoreceptor molecules. Two of the mutant strains, 1224-5/9f and 1224-5/1f, do not possess a stigma and also lack a paraxonemal body, as indicated by fluorescence measurements. The third strain, FB, has a small stigma, but only some cells contain a paraxonemal body. In contrast to the present hypothesis on photoorientation of Euglena, all strains were able to orient with respect to the light direction. However, the mutant strains did not show any orientation at low irradiances. At medium and high irradiances the strains 1224-5/9f and 1224-5/1f oriented perpendicular to the light direction (diaphototaxis) while cells of strains of FB showed partly negative phototaxis and partly diaphototaxis. Diaphototaxis was never observed in the wild type strain. Strains 1224-5/9f and 1224-5/1f showed normal graviresponses compared with the wild type. Astasia longa, a nonphtototactic relative of E. gracilis, as well as strain FB were both negative and positive gravitactic at all culture ages tested. This result confirmed the hypothesis that the paraxonemal body is not directly involved in graviperception.