Paramecium tetraurelia is attracted to acetate and biotin by swimming smoothly and fast up gradients of these attractants, and turning immediately and slowing down when leaving these stimuli. We use a group of mutants, each with a different defect in an identified ion conductance, to show that these two stimuli open different ion channels, and the behaviors that occur upon application of stimulus (on-response) and removal of stimulus (off-response) have different roles in attraction to these two stimuli. The most important parameters for successful attraction to acetate are the on-response behaviors of fast swimming with few turns, and the mutants' behavior suggests that I(K(Ca,h)) is the conductance involved that initiates this behavior. I(K(Ca,h or d)) appears to be important to the on-response in biotin; the results with mutants suggest that the biotin off-response depolarization is initiated by an I(Ca), which can be large enough or close enough to channels to open I(K(Ca,d)), I(Na(Ca)) and I(Mg(Ca)).