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We report on X-ray observations of the solitary 4.8 ms pulsar PSR J0030+0451. The pulsar was one of the last targets observed in 1998 December by the ROSAT PSPC. X-ray pulses are detected on a 4.5 s level and make the source the 11th millisecond pulsar detected in the X-ray domain. The pulsed fraction is found to be 69% +- 18%. The X-ray pulse profile is characterized by two narrow peaks that match the gross pulse profile observed at 1.4 GHz. Assuming a Crab-like spectrum the X-ray flux is in the range fX = (2-3) x 10-13 ergs s-1 cm-2 (0.1-2.4 keV), implying an X-ray efficiency of LX/img1.gif ~ (0.5-5) x 10-3(d/0.23 kpc)2.