The increasing ecological significance of Planctomycetes and the still limited knowledge of this group prompted us to obtain cultured isolates from the sediment of a treatment water recycling tank of a marine fish farm. Presence of strains from this group was assessed in the sediments and water column of the tank. Eleven isolates were obtained from the sediment sample by exploiting Planctomycetes natural resistance to several antibiotics and their capacity to degrade organic matter. Based on morphological characteristics and resistance to antibiotics, Planctomycetes were identified. Their phylogenetic affiliation was confirmed by the sequence analysis of the 16S rRNA gene that revealed the presence of a group of 6 isolates closely related to Rhodopirellula baltica and a cluster of 5 isolates with 97.7-97.9 % of similarity to this species, which probably are a different species of Rhodopirellula. ERIC-PCR profiles showed a higher discrimination within the two groups and allowed the identification of nine different genotypes within the isolated strains. This work corroborates the association of Rhodopirellula spp. with fish farm environments.