none The acceptance of various-aged nymphs of Lygus lineolaris (Palisot de Beauvois) by Leiophron uniformis (Gahan) and the immune response of these nymphs toward the parasitoid were determined. Even the youngest nymphs tested (0-24 h old) showed some encapsulating ability (7.6%), which increased to gt 70% encapsulation by nymphs 72-96h old. Acceptance of nymphs ltoreq 48-h old by the parasitoid females was high (74%) but dropped rapidly as the host ability to resist parasitization increased.