pCAR1 endows carbazole-degrading ability on its host. Three different host Pseudomonas (P. fluorescens Pf0-1, P. resinovorans CA10dm4, and P. chlororaphis IAM1511) were prepared to compare their behaviors in artificial microcosms (sterile soil and water supplemented with carbazole) with previous results for P. putida SM1443 (Shintani et al., Appl. Microbiol. Biotechnol., 80, 485-497 (2008)). We monitored carbazole degradation, bacterial survival, and conjugative transfer of pCAR1. Carbazole degradations were not detected with any of three hosts in the soil microcosms, but were in most of the water microcosms. Degradative efficiency varied host by host. SM1443 is an appropriate host for carbazole degradation in soil microcosms, while CA10dm4 is for water microcosms. pCAR1 transfers were detected only in the water microcosms inoculated with SM1443 or IAM1511 as the host. This is the first report to compare the behaviors of various hosts of the same plasmid in these microcosms.