Rocky ocean bottoms are covered from time to time with small amounts of fine sediments. This material may interfere with the development of germling stages of Macrocystis pyrifera. Tests were performed by introducing sediments before and after spores were dispersed in culture dishes. 10 mg cm-2 of sediment, enough to occlude the surface, prevents spore attachment, greatly reducing the probability of survival. Smothering of established germlings was severe at 108 mg sediment cm-2. Water motion further reduced spore success where sediments were present, probably because of abrasive scour. Mechanisms of natural and human interference with M. pyrifera reproduction are suggested.