A model for the spatial structure of firefly luciferase--ATP--luciferin complex is suggested using the coordinates of unliganded luciferase and the enzyme--substrate complex of the adenylating subunit of gramicidin S synthetase known from the literature. Conformational changes in luciferase can occur during substrate binding resulting in a relative orientation of two luciferase domains similar to that in case of the AMP--phenylalanine--synthetase complex. The model is consistent with data on the physicochemical properties of firefly luciferase and its complexes with the substrates.