The mini Mu element Mu dII1681, which contains the lac operon genes and a kanamycin resistance gene, was inserted in the chromosome of plant growth-beneficial Pseudomonas aeruginosa 7NSK2 to construct a marked strain (MPB1). In MPB1, beta-galactosidase is permanently expressed under the culture conditions used. The MPB1 strain could be recovered with an efficiency of about 100% from a sandy loam soil on 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside medium containing sebacic acid and kanamycin. The limit of detection is about 10 CFU/g of soil. A detailed comparison was made between the wild-type strain 7NSK2 and the Mu dII1681-containing MPB1 strain. The results showed that no genes essential for growth, siderophore production, survival in sterile and nonsterile conditions, plant growth stimulation, or root colonization had been damaged in the MPB1 strain, which means that MPB1 can reliably be used for ecological studies in soil. MPB1 survived well at 4 or 28 degrees C but died off relatively rapidly in air-dried soil or at subzero temperatures. In these conditions, however, the MPB1 strain did not completely disappear from the soil but survived at a very low level of about 100 CFU/g of soil for more than 3 months. This observation stresses the need for very sensitive counting methods for ecological studies and for the evaluation of released microorganisms. Maize was inoculated with MPB1 via seed inoculation or soil inoculation. Upon seed inoculation, only the upper root parts were effectively colonized, while soil inoculation resulted in a complete colonization of the root system.