The research on earthworms on the mainland and islands of the Cuu Long river delta (Mekong delta) were conducted to evaluate the biodiversity and distributional characteristics of this animal. Earthworm samples were collected from the mainland of several islands including Hon Tre, Lai Son, An Son and Phu Quoc. A total of 19700 earthworm individuals were collected from 332 surveyed sites in the period of 2007-2020. As a result, a total 45 species in 11 genera, 4 families (Megascolecidae, Glossocolidae, Moniligastridae, Otochaetidae) were recorded in this region. Of which, Megascolecidae is the most diverse family with 5 genera, 37 species.

The diversity of earthworms descreased from mountainous areas (H' = 3.52; λ = 0.13; 24 species) to islands (H'= 3.40; λ = 0.15; 25 species), and lastly the plain areas (H' = 3.22, λ = 0.16; 23 species). Species diversity, density and biomass of earthworms in rainy season (29 species; H' = 3.48; J' = 0.72; n = 61.11 ind./m2; p = 43.36 g/m2) were higher than that in dry season (15 species; H' = 3.16; J' = 0.81; n = 38.21 ind./m2; p = 16.25 g/m2). Earthworms could be divided into two groups: annual life-cycle (34 species) and unanual life-cycle (8 species). Depth distribution of earthworms depended on eco-morphological characteristics, soil huminity and soil characteristics; Po. elongata was recorded in the depth of 60 cm.