The dtype returned by np.where looks right (int64): >>> import platform >>> platform.architecture() ('64bit', 'WindowsPE') >>> import numpy as np >>> np.__version__ '1.9.0b1' >>> a = np.zeros(10) >>> np.where(a == 0) (array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9], dtype=int64),) -- Olivier