From pea (Pisurn sativum L.) plants with prominent virus symptoms collected in the surroundings of Maglaj, Bosnia, an extremely unstable cucumovirus was isolated. According to the host range, symptom expression and stability in sap the virus was similar to the strains of cucumber mosaic virus (CMV) which infect systemically species from the family Fdbaceae and showed certain properties typical of peanut stunt virus (PSV). The virus reacted specifically with serum against Car and Q strains of CMV (SDI = 3) in crude plant extracts obtained by grinding infected plant tissue in low molarity citrate buffer. It would be possible to detect whether the virus (isolate PY) belongs to CMV or to PSV only in more precise serological tests with purified and stabilized virus preparations.