The deep seismic profile P1 was conducted in northwestern Poland as part of the POLONAISE'97 project. Data acquisition involved 100 mobile seismographs, which recorded seismic waves from ii dispersed chemical controlled explosions along this 300-km-long profile. A variety of seismic modelling techniques were employed to der;ve a velocity model along the profile. This model indicates that the Mesozoic and Palaeozoic sediments are 6 km thick in the northwest and 3 km thick in the southeast, with the main thickness variation taking place in the Palaeozoic succession. An undulating basement surface indicates tectonic activity during deposition of the lower Palaeozoic sequence. The crust is 30-33 km thick along the profile, thickening gradually to the southeast. The P-wave seismic velocities are low (< 6.1-6.2 km s(-1)) down to a depth of 20 km along the whole profile, which we interpret as evidence for their being of sedimentary or volcanic origin. The lower crust has a high P-wave velocity (6.7-7.5 km s(-1)), high vertical velocity gradient and strong, ringing reflectivity. The velocity of the sub-Moho mantle (P-n) is high (> 8.2 km s(-1)). Features similar to the ones below this profile have been interpreted for the area south of the Caledonian Deformation Front in northern Germany and southern Denmark. These similarities indicate that the crust between the southeastern North Sea and Poland has a common origin. We therefore suggest that the deep crust beneath the profile is not of Variscan origin, despite its location south of the Variscan Deformation Front. We propose that Variscan tectonism in the area was thin-skinned, while the deeper crust may originate from the Caledonian microcontinent East Avalonia or possibly from Baltica.