The study deals with a topic in Czech word formation, namely with the determination of the direction of formation in pairs of a suffixless noun and a related verb (e.g. skok "jump" - skočit "to jump", sůl "salt" - solit "to salt"). While corpus frequency is an unreliable criterion due to formal-meaning asymmetries and meaning shifts, alternations in the prefix and root morphemes of the studied nouns and verbs, as well as the way of formation of the aspectual counterparts, are useful clues - the latter criterion can be applied to both domestic and foreign verbs, including neologisms.