Luciferases from copepodsMetridia longaandGaussia princepsare successfully used as bioluminescent reporters for in vivo and in vitro assays. Here, we report the minimal sequence of copepod luciferases required for bioluminescence activity that was revealed by gradual deletions of sequence encoding the smallest MLuc7 isoform ofM. longaluciferase. The single catalytic domain is shown to reside within the G32-A149 MLuc7 sequence and to be formed by both non-identical repeats, including 10 conserved Cys residues. Because this part of MLuc7 displays high homology with those of other copepod luciferases, our suggestion is that the determined boundaries of the catalytic domain are the same for all known copepod luciferases. The involvement of the flexible C-terminus in the retention of the bioluminescent reaction product in the substrate-binding cavity was confirmed by structural modeling and kinetics study. We also demonstrate that the ML7-N10 mutant (15.4 kDa) with deletion of ten amino acid residues at the N-terminus can be successfully used as a miniature bioluminescent reporter in living cells. Application of a shortened reporter may surely reduce the metabolic load on the host cells and decrease steric and functional interference at its use as a part of hybrid proteins.