Abstract We present the results of an experiment on Cross Seamount (18°40′N, 158°17′W) in which basalt, ferromanganese-oxide and CaCO 3 substrates were deployed for 19 months. An experimental block design, of identical sets of well-characterized substrates arranged on three distinct panels, was used. Alterations resulting from exposure at 800 m water depth were documented by analytical Scanning Electron Microscopy. Agglutinated rhizopods and irregularly shaped chambers occupied an average of 37% of the basalt surfaces, and 13 and 20% of the ferromanganese-oxide and CaCO 3 substrates, respectively. This coverage is high, relative to that observed on dredged ferromanganese nodules and crusts. High coverage can be attributed to the low abundance of other agglutinated and calcareous foraminifera. Metal-rich deposits composed of BaS, AlSi, Mn and Fe were common on substrate surfaces. Barite (BaS) particles, which originated in the water column, were observed attached to the substrate by agglutinated foraminiferal tests. Fe-enriched AlSi deposits were often adhered to the substrate. These deposits appeared to have been produced by benthic rhizopods, and may persist on the seafloor over time. Fe-coated bacteria-like colonies were numerically scarce, but covered large areas. Mn-oxide precipitates, also bacteria-like, were numerically abundant on ferromanganese-oxide substrates, but never observed on other substrates. The occurrence and morphological features of these Fe- and Mn-oxide coatings reinforce the idea that bacteria play an important role in the accretion of ferromanganese-oxides on the seafloor.