The gravitropic response of Pellia epiphylla (L.) Corda sporophytes was analyzed using digitized images from infrared video recordings. Lateral transport of radioactively labeled auxin was followed using agar block and liquid scintillation techniques. Upward gravitropic curvature of horizontally oriented sporophytes was detected within 50 to 60 minutes. Growth rates on the top sides of sporophyte setae dropped from 0.53 to 0.29 mm hr⁻¹, and rose on the bottom from 0.53 to 0.88 mmAhr⁻¹. ¹⁴C-labeled indoleacetic acid (IAA) applied in donor blocks moved preferentially toward the lower side of setae in horizontally oriented sporophytes, with three-fold polarity established after 40 minutes. Negligible movement of label from blocks was detected in setae of vertically positioned controls tested after the same interval. These observations are consistent with a mechanism for gravitropism in Pellia that involves lateral redistribution of IAA.