Calcium (Ca) movement across tips of primary and lateral roots of Phaseolus vulgaris was determined by applying 45Ca2+ to one side of the root and collecting radioactivity in an agar receiver block on the opposite side of the root. The ratios of cpm in receiver blocks on the bottom of primary roots : cpm in receiver blocks on the top of the primary roots were 1.87 and 2.47 after 1 and 2 hr, respectively. This polar transport of Ca across tips of primary roots correlated positively with a graviculture of 43 degrees after 2 hr. The ratio of cpm in receiver blocks on the bottom of lateral roots : cpm in receiver blocks on the top of lateral roots was 1.20 after 2 hr. The decreased polar movement of Ca across tips of lateral roots correlated positively with lateral roots being nongraviresponsive. These data 1) support the suggestion that gravistimulation induces polar movement Ca toward the lower side of tips of primary roots, and 2) suggest that the reduced polar movement of Ca across tips of lateral roots may be involved in uncoupling gravistimulation from gravicurvature in lateral roots.