Open clusters with Hipparcos I. Mean astrometric parameters

New memberships, mean parallaxes and proper motions of all 9 open clusters closer than 300 pc (except the Hyades) and 9 rich clusters between 300 and 500 pc have been computed using Hipparcos data. Precisions, ranging from 0. 2 to 0.5 mas for parallaxes and 0.1 to 0.5 mas/yr for proper motions, are o f great interest for calibrating photometric parallaxes as well as for kine matical studies. Careful investigations of possible biases have been perfor med and no evidence of significant systematic errors on the mean cluster pa rallaxes has been found. The distances and proper motions of 32 more distan t clusters, which may be used statistically, are also indicated.