Surface morphology and nanostructure of the ultrahigh molecular weight polyethylene fiber Spectra 900 were examined by atomic force microscopy underwater, with height and lateral force registration. Non-uniform surface morphology, where the bundles of microfibres are covered by interwoven randomly oriented fibrils ("hairs") and twisted ribbons, was registered in the large scale images. The nanofibril elements detected in the bundles and in the "hairs" are 5 - 8 nm in width. Periodical stripes found by lateral force measurement on individual fibrils (long period) were assigned to sequences of crystalline and less-ordered regions. The repeat distance of the long period varies from place to place being in the range of 10 - 20 nm. The complexity of the polyethylene fiber structure is supported by visualizing large crystallites, which were found in the ribbons split from the fiber surfaces.