It is described biologically pure bacterial cultures of mutant strains of Pseudomonas fluorescens, which produces large amounts of alginate. The alginate may contain a certain determined content of mannuronate and guluronate residues, possible presence and determined level of acetyl groups in the alginate, and a desired molecular weight of the alginate. Also high yielding mutants with regulation of alginate production, is described. The invention further provides methods for producing new mutant strains of Pseudomonas fluorescens and variants thereof, and use the resulting strains in alginate production.