SUMMARYPotato mop‐top virus (PMTV) was best detected in field soils by air‐drying them for more than a week before remoistening and growing seedlings ofNicotiana tabacumorN. debneyifor a 6–10 week period. Infection ofN. tabacumwas assessed by inoculating sap from roots and shoots toChenopodium amaranticolor.Similar inoculations fromN. debneyiwere far less convenient for detecting PMTV than recording leaf symptoms, but slightly more efficient.Air‐dry soil retained PMTV infectivity for 9 months, when passed through a 50 μ sieve or when diluted with 103but not 104parts of steamed soil. Tobacco seedlings were not infected when their roots were steeped in PMTV‐containing tobacco sap. Infective soils containedSpongospora subterranea, spore balls of which resisted air‐drying for more than a year and passed a 50 μ sieve. Roots of susceptible seedlings were infected with PMTV when exposed to spore balls ofS. subterraneataken from powdery scabs on PMTV‐infected potato tubers, or to suspensions obtained by steeping, in nutrient solution, roots infected with virus‐carrying cultures ofS. subterranea.Plants in several families were hosts ofS. subterranea, but probabilities of infection when exposed to spore balls differed greatly between families and only species of Solanaceae were good hosts. The ten species infected with PMTV when grown in infective soil or when exposed to spore balls ofS. subterraneataken from PMTV‐infected potato tubers are all members of this family. PMTV seems to be carried internally inS. subterraneaspore balls and survived in them for at least a year.PMTV was transmitted byS. subterraneato Arran Pilot potato, causing yellow blotches in some leaves and spraing in many tubers. However, when newly infected with PMTV in the field, not all Arran Pilot tubers developed spraing. Also, although many spraing‐affected or symptomless but PMTV‐infected tubers carried PMTV‐containing spore balls ofS. subterranea, powdery scabs were rarely found near the centres of the rings of primary spraing. PMTV became established in virus‐free soil when PMTV‐infected tubers carryingS. subterraneawere planted as ‘seed’ but not when virus‐free tubers bearing powdery scabs were used. 5.subterraneaseems the main, and possibly the only, vector of PMTV in the soils examined.S. subterraneadid not transmit potato aucuba mosaic virus from p