Microtubules are polymers composed of α-β tubulin heterodimers that form 12 to 15 protofilaments and assemble into microtubules . Microtubules are dynamic structures that have periods of both growth and shrinkage by addition and removal of subunits from the polymer . Microtubules stochastically switch between periods of growth and shrinkage, termed dynamic instability . Dynamic instability is coupled to the GTPase activity of the β-tubulin subunit of the tubulin heterodimer . Microtubule dynamics are regulated by microtubule associated proteins (MAPs), that interact with microtubules to regulate dynamic instability . MAPs in budding yeast have been identified that bind microtubule ends (Bim1), that stabilize microtubule structures (Stu2), that bundle microtubules by forming cross-bridges (Ase1), and that interact with microtubules at the kinetochore (Cin8, Kar3, Kip3) [6-10]. IRC15 was previously identified in four different genetic screens for mutants affecting chromosome transmission or repair [11-14]. Here we present evidence that Irc15 is a microtubule associated protein, localizing to microtubules in vivo and binding to purified microtubules in vitro. Irc15 regulates microtubule dynamics in vivo and loss of IRC15 function leads to delayed mitotic progression, due to failure to establish tension between sister kinetochores.