Abstract. (1) The immigration of spiders into the Lauwerszeepolder (constructed in 1969) was studied during four years in four areas. The spider fauna was sampled weekly and 65 000 specimens of eighty-four species were caught using strip traps, window traps, simple pitfall traps and fences.

(2) About sixty species from all kinds of habitats were caught rarely; nineteen species were caught in numbers that suggest that they had established populations in the study areas.

(3) The four successful pioneer species, plus two marsh-dwelling species, were most abundant during 1969 and 1970. The 1971 catch was dominated by species from saline habitats; they became less abundant in 1972 when species from non-saline habitats increased. Only halotolerant species established populations.

(4) No evidence was found that dispersing spiders select certain areas. The differences between the spider fauna of the differennt study areas arise from degrees of success in colonization by immigrants. Pioneer species are least influenced by the abiotic environment.

(5) A greater aeronautic dispersal power is found in species inhabiting unstable habitats than in species from stable habitats.

(6) Aeronautic activity in adult linyphiids (s.l.) is not restricted to a special season, but is related to definite phases of the phenology.

(7) Male and female erigonids are equally active in aeronautic behaviour. In erigonids and linyphiids intense aeronautic activity coincides with great ground activity; on the ground males are more active than females.