ABSTRACT: Thraustochytrids and aplanochytrids, belonging to the Labyrinthulomycetes of the Kingdom Stramenopila, have been frequently reported to occur as parasites or symbionts in a number of coastal invertebrates. Given the fact that these protists are widespread in coastal and oceanic waters, it is possible that they are also associated with pelagic zooplankton. This study examines their occurrence in zooplankton from equatorial waters of the Indian Ocean. A total of 171 of 2100 individual specimens of zooplankton yielded Labyrinthulomycetes in culture, the colony morphology and/or life cycles of which corresponded to aplanochytrids. Small-subunit ribosomal DNA (SSU rDNA) sequence analysis of 8 of the isolates placed them in a distinct clade among aplanochytrids, but closest to Aplanochytrium yorkensis or A. kerguelensis. The 8 isolates were further segregated into 2 clusters, corresponding to isolates obtained from 2 different seasons. Biotinylated probes for 2 isolates of A. kerguelensis from the 2 different seasons were developed based on internal transcribed spacer (ITS) sequences. In situ hybridization (ISH) of zooplankton using the probes with streptavidin-alkaline phosphatase showed that aplanochytrid cells fed to copepods were grazed and could subsequently be detected in the animals. ISH on natural samples of zooplankton yielded a positive but diffuse reaction in copepods, while cells resembling aplanochytrids were detected within chaetognaths. ISH using streptavidin-peroxidase conjugate lent further support for their presence within chaetognaths. This study suggests the predominant occurrence of A. kerguelensis in association with mesozooplankton of the equatorial Indian Ocean, particularly with chaetognaths. Further studies are suggested to determine whether genetically distinct populations of A. kerguelensis are associated with chaetognaths in oceanic waters and if such an association is parasitic, mutualistic or commensalistic.