Bioluminescence by the squid Euprymna scolopes requires colonization of its light organ by the symbiotic luminous bacteriumVibrio fischeri . Investigation of the genetic determinants underlying bacterial symbiotic competence in this system has necessitated the continuing establishment and application of molecular genetic techniques in V. fischeri. We developed a procedure for the introduction of plasmid DNA into V. fischeri by electroporation, and isolated a mutant strain that overcame the apparent restriction barrier between V. fischeri and Eschirichia coli. Using the technique of electroporation in combination with that of gene replacement, we constructed a non-luminous strain of V. fischeri (Δlux A: :erm). In addition, we used the transducing phage rp-1 for the first time to transfer a chromosomal antibiotic resistance marker to another strain of V. fischeri. The lux A mutant was able to colonize E. scolopes as quickly and to the same extent as wild type. This result suggested that, at least during the initial stages of colonization, luminescence per se is not an essential factor for the symbiotic infection.