G.A. MONTEIRO, A.M. FIALHO, S.J. RIPLEY AND I.SÁ -CORREIA. 1992. The electrotransformation of gellan-gum producing or non-producing strains of Pseudomonas elodea (Gel+ or Gel-) was optimized with respect to growth stage, cell and DNA concentrations and pulse parameters. This technique proved to be a valuable alternative to conjugal mating to search for complementation of gellan mutations for cloning the gellan genes. The electrotransformation efficiency of Gel+ or Gel- strains was similar. The transformation of smaller plasmids was more efficient than that of larger plasmids, and recombinant plasmids with sizes larger than 35 kb, when extracted from Escherichia coli DH1, were not transformable at detectable frequency. This was partially related to the modification/restriction system active in the recipient cells.