  We present the results of ROSAT and XMM-Newton observations of the recurrent
ultraluminous X-ray source (ULX) NGC 253 ULX1. This transient is one of the few
ULXs that was detected during several outbursts. The luminosity reached 1.4
10^39 erg s^-1 and 0.5 10^39 erg s^-1 in the detections by ROSAT and
XMM-Newton, respectively, indicating a black hole X-ray binary with a mass of
the compact object of >11 M_sun. It is highly unlikely that this source is a
foreground source or a background AGN. In the ROSAT detection NGC 253 ULX1
showed significant variability, whereas the luminosity was constant in the
detection from XMM-Newton. The XMM-Newton EPIC spectra are well-fit by a
bremsstrahlung model (kT=2.24 keV, N_H=1.74 10^20 cm^-2), which can be used to
describe a comptonized plasma. No counterpart was detected in the optical I, R,
B, NUV and FUV bands to limits of 22.9, 24.2, 24.3, 22 and 23 mag,
respectively, pointing at a X-ray binary with a low mass companion.
