Colorado potato beetle is the most important potato pest. Because of the problem of resistance development to chemical insecticides, other solutions are required. Entomopathogenic nematodes have been proven effective in suppressing insects of order Coleoptera but their effectiveness on Colorado potato beetle in Croatia has not been sufficiently explored. This work investigated the efficacy of nematode species Steinernema feltiae and Steinernema carpocapsae on overwintered adult Colorado potato beetles in field. Both nematodes were applied in 2018 in three doses by watering the soil. To monitor the dynamics of the emergence from the soil and their numbers, entomological cages were used for one month in the field. In the variants in which species S. feltiae was used, Colorado potato beetle overwintered adults were recorded from 19 April until 10 May. In the variants in which species S. feltiae was applied, the emergence of beetles was somewhat earlier (16 April) and lasted ten days shorter (until 30 April) than in variants treated with S. feltiae. On control variant, the emergence was recorded from 13 April till 7 May. The results of efficacy showed the satisfactory efficacy of both nematodes on overwintered Colorado potato adults. Species S. feltiae had an efficiency of 79.03% to 100.00%, while species S. carpocapsae efficacy was between 77.32% and 96.22%. Both nematode species were most effective in the highest doses administered. The efficacy at recommended doses was also satisfactory (96.22% for S. feltiae; 77.32% for S. carpocapsae). The highest efficacy for both nematodes was achieved one month after the treatment. Due to its advantages over the chemical insecticides and the ever-increasing resistance problem, entomopathogenic nematodes have a great potential for use in the biological control of Colorado potato beetle in the future.