Ribosomal ITS2 DNA fragments were sequenced from four Peristenus species, two Leiophron species, and two Lygus species. Specific primers for polymerase chain reaction (PCR) amplification were designed from ITS2 DNA sequences to separate each species from the others. Using this molecular approach, we were able to determine whether Lygus hesperus Knight and Lygus lineolaris (Palisot de Beauvois) were parasitized by Peristenus and Leiophron parasitoids. The PCR technique was very sensitive and could detect Peristenus stygicus Loan DNA at a concentration of 0.01 pg/μl or 7.5 × 10⁻⁷ wasp DNA equivalents. Detection of P. stygicus eggs confirmed that early detection of parasitoids was possible. Parasitoid DNA was readily recovered from all L. hesperus nymphs that were parasitized by a single P. stygicus after 1 h of contact between the parasitoid and putative hosts. This study demonstrates the effectiveness of a molecular technique for detecting parasitoids developing inside their hosts.