The seagrass and macroalgal vegetation of Gazi Bay (at approximately 50 km south of Mombasa) have been studied by means of 88 releves along 7 transects. Correlation between the distribution of the seagrasses and some abiotic factors (particle size fractions, chemical composition of the substrate) is not well marked. Nevertheless a general zonation and succession of seagrasses could be established: 1. a transition zone between the mangal and the seagrass beds is covered by Boodleopsis pusilla; 2. the pioneer association Halophila ovalis + Halodule wrightii forms low sandy bumps at the upper limit of the seagrass beds, but also occurs in the whole midlittoral where sandlayers have recently been accumulated (e.g. on coral platforms); 3. the climax vegetation of the intertidal zone seems to be Thalassia hemprichii which sometimes is associated with Cymodocea rotundata and C. serrulata, certainly in deeper pools and close to low water mark; Halimeda opuntia, Gracilaria salicornia and G. corticata are also frequent in this vegetation type; 4. from low water at neap tide downwards patches of monospecific Enhalus acoroides vegetation can also occur; 5. from mean low water down to approximately -1 m mixed meadows of Thalassia, C serrulata, C rotundata and Halodule uninervis are well developed; the seaweeds Halimeda macroloba and Avrainvillea obscura are also typical for this zone; locally patches of Syringodium isoetifolium grow on small bumps and Halophila stipulacea grows as a pioneer on bare sand; 6. from -1 m downwards the whole lagoon is covered by homogeneous, monospecific Thalassodendron ciliatum meadows, locally replaced by E. acoroides.