We studied the occurrence and dynamics of transparent exopolymer particles (TEP) and 4'-6'-diamidino-2-phenylindole (DAPI)-stained microaggregates (MA) and the bacterial colonization of the latter by fluorescence in situ hybridization with rRNA-targeted oligonucleotides in Lake Constance, Germany, from June to December 1996. MA were categorized into the size classes 8 to 20, 20 to 60 and > 60 µm. Total abundance of TEP varied between 71 and 2554 ml -1 and that of MA between 310 and 1526 ml -1 with the largest proportion in the size class 8 to 20 µm (140 to 959 ml -1). Numbers of TEP and of MA > 60 µm, never exceeding 100 ml -1 , decreased with depth and were correlated to the biomass of total phytoplankton, diatoms, chrysophytes and dinoflagellates. Numbers of the smaller-sized MA did not systematically decrease with depth. Four types of DAPI-stained MA were identified: aggregates composed of bacterial and of cyanobacterial colonies, aggregates fluorescing yellow (DAPI yellow particles), and phytoplankton and zooplankton fragments fluorescing faintly blue after DAPI staining. The latter and DAPI yellow particles were most abundant. The majority of MA was densely colonized by bacteria. At least 20% of the DAPI-stainable cells were detected by the Bacteria-specific probe EUB338 and a mean between 44 and 56% of the various types of MA. β-Proteobacteria dominated aggregates composed of remains of diatoms and often also on fragments. On DAPI yellow particles bacteria of this phylogenetic lineage and the Cytophaga/Flavobacteria cluster were equally abundant. γ-Proteobacteria usually constituted only minor proportions except on fragments of Dinobryon spp. and of zooplankton origin on which they constituted up to 62% of the DAPI-stainable cells. α-Proteobacteria were not detected at all. Together, all group-specific probes constituted > 95% of the bacteria detected by the EUB338 probe on MA except for bacterial aggregates on which only 59% were detected. The different spatio-temporal dynamics of TEP and MA and the bacterial colonization of the latter suggested that TEP and MA > 60 µm were early decomposition products mainly of diatoms and other algae whereas smaller MA were more degraded stages of detrital particulate organic matter.KEY WORDS: Microaggregates · Transparent exopolymer particles · Bacteria · In situ hybridization · Lake ConstanceResale or republication not permitted without written consent of the publisher