Several growth parameters associated with the phytochrome-mediated shade avoidance syndrome have been measured in seedlings and mature plants of a wild-type and a hy3 mutant of Arabidopsis thaliana deficient in phytochrome B. Growth parameters were compared in plants grown in either white light (high red:far-red [R:FR] ratio) or white light plus added far-red (FR) light (low R:FR ratio). Wild-type Arabidopsis exhibited increased hypocotyl and petiole extension under a low, compared with a high, R:FR ratio. The hy3 mutant did not respond to low R:FR ratio by increase in hypocotyl or petiole length. Extension growth of wild-type plants was stimulated by brief end-of-day FR pulses, but similar treatment had no effect on extension growth of hy3 mutant plants. However, some responses to low R:FR ratio seen in the wild-type plants were also evident in the hy3 mutants. The number of days to bolting, the developmental stage at bolting, the leaf area, and the specific stem weight (weight per unit of length) all decreased in the wild-type and hy3 seedlings in response to low R:FR ratio. Low R:FR ratio caused a larger decrease in leaf area and specific stem weight in the mutant seedlings than in wild-type seedlings. The effects of low R:FR ratio on leaf area and specific stem weight were opposite to those of the hy3 lesion, which resulted in increased leaf area and specific stem weight in comparison with the wild type. Both leaf area and specific stem weight responses to low R:FR ratio also were unchanged in the ein mutant of Brassica rapa, known to be deficient in phytochrome B. These responses represent components of the shade-avoidance syndrome, and, consequently, the results indicate that phytochrome B cannot be solely responsible for the perception of R:FR ratio and the induction of shade-avoidance responses. The hypothesis is proposed that different phytochromes may be responsible for the regulation of extension growth and the regulation of lateral or radial expansion.