Microtubule dynamic instability underlies many cellular functions, including spindle morphogenesis and chromosome movement. The role of guanosine triphosphate (GTP) hydrolysis in dynamic instability was investigated by introduction of four mutations into yeast beta-tubulin at amino acids 103 to 109, a site thought to participate in GTP hydrolysis. Three of the mutations increased both the assembly-dependent rate of GTP hydrolysis and the average length of steady-state microtubules over time, a measure of dynamic instability. The fourth mutation did not substantially affect the rate of GTP hydrolysis or the steady-state microtubule lengths. These results demonstrate that the rate of GTP hydrolysis can modulate microtubule length and hence dynamic instability.