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 DEC-98 by the ROSAT PSPC .
X-ray pulses are detected on a 4.5 \sigma level and make the source the 11 ^ { th } millisecond pulsar detected in the X-ray domain .
The pulsed fraction is found to be 69 \pm 18 \% .
The X-ray pulse profile is characterized by two narrow peaks which match the gross pulse profile observed at 1.4 GHz .
Assuming a Crab-like spectrum the X-ray flux is in the range f _ { x } = 2 - 3 \times 10 ^ { -13 } erg s ^ { -1 } cm ^ { -2 } ( 0.1 - 2.4 keV ) , implying an X-ray efficiency of L _ { x } / \dot { E } \sim 0.5 - 5 \times 10 ^ { -3 } ( d / 0.23 \mbox { kpc } ) ^ { 2 } .