1. To determine the ontogeny plus the intensity and spectral characteristics of polarotaxis in larvae of the crab, Rhithropanopeus harrisi, swimming paths in relation to the e-vector direction of linearly polarized light were monitored and recorded with a closed circuit video system. 2. All four zoeal stages were examined. Only stages II and III responded with significant directional swimming (stage II, perpendicular to the e-vector, stage III, parallel to the e-vector; Figs. 3 and 4). Stages I and IV appeared indifferent to the polarization plane (Figs. 2 and 5). Swimming under unpolarized light was consistently random (Table I). 3. Polarotaxis disappeared following dark adaptation of stage II larvae (Fig. 6). 4. The lower intensity threshold at 499-nm light for polarotaxis was found to be between 10-4 and 10-3 W/m2, while the upper threshold is in the range 10-1 to 2 x 10-1 W/m2 (Figs. 3 and 4). 5. Examination of selected wavelengths from 420-620 nm yielded greatest responsiveness at 499 nm (Fig. 7). A rise evident at 420 nm suggests a secondary maximum in the violet. 6. Stage II larvae showed significant positive phototaxis under depolarized light when a collar of alternating black and white quadrants surrounded the experimental vessel. With the e-vector of polarized light aligned parallel to the white sectors, phototaxis was reduced by polarotactic swimming into the black sectors (approximately perpendicular to the e-vector). 7. A comparison with phototaxis (Forward and Costlow, 1974) provides two items of evidence that polarotaxis and phototaxis are indeed separate responses. First, phototaxis is essentially unchanged during larval development, while polarotaxis appears only at stages II and III. Secondly, the responses have different optimum intensities.