The blazar type active galaxy Mkn 421 was observed during an active phase in January and February 2001 with the X-ray satellite RXTE and with the ground-based HEGRA system of TeV Cherenkov telescopes. Observations were coordinated to allow for simultaneous data taking. Both instruments observed time-dependent variations of the flux and the spectral shape in X-rays and TeV γ-rays. Variability and correlations in the TeV and X-ray bands are observed on timescales of hours.