The spatfall patterns of the oyster,Crassostrea cucullata(Born 1778) and the barnacle,Balanus amphitrite(Darwin 1854) on cemented coconut shells were studied in relation to shore levels and monsoon seasons in East Africa. Both oysters and barnacles settled most frequently above the Mean Tide Level (MTL). About 75.6% of the oyster and 96.9% of the barnacles occurred above the MTL. Thus below MTL, the oyster spat had a better niche advantage over the barnacles. However, at the latter levels the oysters faced heavy competition from fouling organisms, which were mostly spirobids, serpulids, bryozoans, hydrozoans and sponges. The spat of both oysters and barnacles showed seasonality in their settlement. About 93.1% of the total oyster spat settlement occurred in the South East Monsoon (SEM) and the number of spat at peak settlement was 12.6 times the total spat settlement in the North East Monsoon (NEM). As regards the barnacles, 86.4% of the total barnacle spat settlement occurred in SEM and the number of spat in the peak settlement was 3.4 times the total settlement in NEM. Oysters and barnacles on the cemented coconut shells had a wider vertical distributional range than those found on mangrove trees.