The details of the dislocation morphologies in iron single crystal during stage I plastic straining with and without the presence of a concurrent supply of hydrogen have been studied. Hydrogen was shown to greatly enhance the tendency of strain localization especially near hydrogen-containing spherical inclusions. Three different strongly operating slip systems have been identified in the localized dislocation tangled structure, in contrast to the surrounding matrix or the region around the same particles in hydrogen free specimens, where one primary slip system predominated. It was suggested that hydrogen not only enhances the mobility of primary screw dislocations, but also affects the local stress and strain associated with particles, to promote dislocation generation on those slip systems associated with twin formation. If there is no competing slip system, the twinning-sense dislocations catalyzed by hydrogen can lead to the formation of crystallographic identifiable twins. If, on the other hand, a strong anti-twinning primary slip operates, as in the present study, the twinning process is suppressed and the associated dislocations interact locally with the primary system to promote a dislocation tangled structure. Hydrogen-induced microcrack formation was found to have an orientation similar to the strain localization band, suggesting that a direct correspondence exists between the presence of hydrogen-induced strain localization bands and the initiation of hydrogen-induced cracks.