The green fluorescent protein gene (gfp) was introduced into a p-nitrophenol-metabolizing strain of Moraxella sp. by chromosomal integration. The gfp-marked transformants, designated Moraxella sp. strains G21 and G25, exhibited green fluorescence under UV light. Molecular characterization by PCR and Southern hybridization showed the presence of gfp in both transformants. Both transformants and the parent strain degraded 720 μM of p-nitrophenol with nitrite release within 4 h after inoculation in minimal medium supplemented with yeast extract. Transformants degraded up to 1440 μM p-nitrophenol and mineralized about 60% of 720 μM p-nitrophenol, both in broth and in soil, to the same extent as the parent strain. Insertion of gfp did not adversely affect the expression of p-nitrophenol-degrading genes in the transformants. Survival studies indicated that individual green fluorescent colonies of transformants can be detected up to 2 weeks after inoculation in soil. These marked strains could be of value in studies on microbial survival in the environment.