The pre-penetration sequence involving zoospore taxis, encystment, cyst germination and germ-tube orientation was studied by video microscopy when roots of wheat, cress or tomato were placed in suspensions of zoospores of Pythium aphanidermatum (Edson) Fitzp. The sequence was also studied in vitro, to identify the responsible factors.Several amino acids, sugars or volatile compounds elicited one of more stages of the sequence, but only glutamic acid and aspartic acid elicited all stages and thus could account for the observed behaviour on roots. Polyuronates of root surface slime also may be needed for encystment and orientation of spores, because germ-tubes emerged towards roots from a fixed point near the former position of the water-expulsion vacuole. However, identical patterns of zoospore accumulation and encystment on untreated roots and roots encased in alginate gel indicated that neither polyuronates nor specific sugar residues of root slime caused localized encystment on roots.Based on these results, a 'simplest case' model of the pre-penetration sequence on roots is proposed.