Abstract. Nipah worm (Namalycastis rhodochorde) is one of the polychaeta group lived in the Nypha fruticans mangrove sediment in West Kalimantan. It has potentialy to be cultivated because it has a high economic value in aquaculture. Feed aspect is an important part of its cultivation, through improving the quality of feed by using probiotics isolated from its natural habitat, such as actinomycetes that are able to produce secondary and primary metabolites. These metabolites are expected to increase immunity and biomass of worms. The purpose of this study were to isolate and determine the composition and characteristics of actinomycetes derived from mangrove sediment, gastrointestinal and fecal pellets of nipah worm that be potential as probiotic for feed formulation. Isolation and characterization of actinomycetes was conducted by pour plate method on SCA, ISP2, ISP3, ISP4 and ISP5 by determining morphological characteristics of colonies, cell structure, and biochemical tests based on Bergey’s Determinative of Bacteriology. The results showed that twelve actinomycetes had been isolated. Six isolates from mangrove substrate, four isolates from fecal pellets and two isolates from the gastrointestinal tracts. All isolates were closely related to Streptomyes genera.

Keywords: Actinomycetes, Namalycastis rhodochorde, nipah worm, Streptomyces.