Phototaxis in wild-type Chlamydomonas cells is probably the result of four single photoresponses in the flagella, inverse in the cis (near the eyespot) and in the trans flagellum and also inverse by step-up (increase of) and step-down (decrease of) light stimulation as experienced by the eyespot during rotation of the cell [Rüffer and Nultsch, 1991: Cell Motil. Cytoskeleton 18:269-278]. Two inverse sets of the four responses are supposed to be the cause for positive and negative phototaxis. The relevant flagellar responses consist of shifts of the front amplitude of the breaststroke beats. As single flagellar responses cannot be called "phototactic" they are termed "breaststroke flagellar photoresponses." The mutant strain ptx1 is defective in phototaxis but displays photoshocks [Horst and Witman, 1993: J. Cell Biol. 120:733-741]. Analysis of flagellar beat patterns in high-speed records of ptx1 cells held on micropipettes shows that breaststroke flagellar photoresponses exist in this mutant in spite of the loss of phototaxis. It is the cis/trans differentiation that is lost in ptx1: both flagella always respond in the same way and not inversely as in wild-type cells. Equal shifts of beat amplitude cannot cause a turn of the cell, which explains why phototaxis is not seen in ptx1 and supports the model suggested for positive and negative phototactic steering. In wild-type cells front amplitude changes are connected with beat period changes, which also occur in ptx1 cells and suggest that both flagella respond like wild-type trans flagella. Divergencies in the shock response of ptx1 cells, beat period reduction, and coordination changes may support the notion that cis flagellar specialization is lost and that ptx1 possesses, so to speak, two trans and no cis flagellum. Therefore, the mutant strain ptx1 might be useful for studying molecular and functional peculiarities of the two flagella.