Mechanisms of particle selectivity controlling agglutination within the agglomerated tintinnid genus Stenosemella spp. Jörgensen were studied during two iron fertilisation experiments (EisenEx and EIFEX) conducted in the Southern Ocean in austral spring 2000 and late austral summer to early autumn 2004. Representative SEM pictures of Stenosemella spp. loricae were taken on the day of fertilisation, during the middle and at the end of both experiments. Whereas during EisenEx particles used for agglutination were unambiguously dominated by coccoliths of Emiliania huxleyi (Lohmann) Hay & Mohle, agglutinated particles during EIFEX mainly consisted of broken diatom frustules (BF) of heavily silicified species. This observation is supported by the ratio of E. huxleyi (Eh)/BF abundances in the water column during both experiments. During EisenEx we observed an Eh/BF ratio which was an order-of-magnitude higher compared to the EIFEX experiment. Thus, our results clearly indicated that particle availability seems to be the driving mechanism in the agglutination of Southern Ocean tintinnids. Furthermore, possible implications for the vertical flux of agglutinated biogenic particles to deep ocean waters and sediments are discussed. © 2007 Elsevier GmbH. All rights reserved.