1. The pollination system in 4x blackberry was studied from vicinism (cross-pollination with neighbouring plants) in a row of Merton Thornless plants situated between rows of thornyRubus, using glandularv. eglandular cotyledons as markers. 2. Average hybridization was 17%, ranging from 5 to 32% in different flower samples. End plants were 25% outcrossed and those within the row were 15%, the difference being partly due to intra-pollination within the row. Terminal flowers were 19% outcrossed but lateral flowers only 16%; flowers on south-facing inflorescences had 20% and those facing north 15%. These differences were statistically significant. 3. Average first-year germination was 45%, with a range from 18 to 66% in different fruit samples. End plants had 50% germination and plants within the row had 44%; terminal fruits had 42% and laterals 49%. These differences were statistically significant. South and north facing fruits had similar germinations. 4. These results show that micro-factors affecting vicinism in blackberry include flower position, orientation of inflorescences and a plant's position relative to others. They constitute thepollination milieu and act besides major factors of spatial and temporal isolation.