The rotational dynamics of CO single molecules solvated in small He clusters (CO@He-N) has been studied using reptation quantum Monte Carlo simulations for cluster sizes up to N=30. Our results are in good agreement with the rotovibrational features of the infrared spectrum recently determined for this system and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nanodroplet regime, which has not been measured so far. (C) 2004 American Institute of Physics.