SUMMARY: Intraspecific and interspecific transformation in Pseudomonas has been investigated. Two isoleucine+valine requiring mutants of Pseudomonas aeruginosa have been transformed to prototrophy. By using gelatin liqui-faction as a marker character it was observed that two strains of P. fluorescens, a phytopathogen P. mangiferaeindicae and a strain of Pseudomonas originally isolated from fish were transformable. When P. solanacearum was used as a donor, the transformation frequency of P. mangiferaeindicae was very high. DNA from fish Pseudomonas I was capable of transforming two strains of P. fluorescens and P. mangiferaeindicae in addition to fish Pseudomonas 21. When P. fluorescens was treated with the DNA from P. aeruginosa transformation frequency was lower; the capacity of pigment production, however, could not be transferred to any of the strains. The highest transformation frequency was obtained towards the end of the log phase of growth of the receptor bacteria, with a DNA concentration of about 10 μg./ml. Optimum temperature for transformation was 25–30° and agitation and starvation in diluted media increased the number of transformants. The significance of the observations has been discussed from the point of view of relationships among the strains and species studied.