Development of new, more aggressive isolates of Phomopsis helianthi in Serbia and Montenegro has been investigated. Two isolates of Phomopsis helianthi, Yu 4 and Yu 12, had been found on the stem of NS-H-45, a highly tolerant sunflower hybrid. The isolate Yu 4 was isolated in the northern part of the Vojvodina Province, a region where Phomopsis typically occurs. The isolate Yu 12 comes from the southern part of the Vojvodina Province, a region along the Danube River, which suffered an intensive outbreak of Phomopsis in 1997. A preliminary study of the isolates showed that these two isolates were signifi cantly different for radial growth in Petri dish. A test was thus made with recombinant inbred lines in the F9 generation developed from the cross of LR 4-17 (inbred line with high tolerance to Phomopsis) and HA-89 (susceptible inbred line). Experiments were conducted in the greenhouse, inoculating plant leaf tips with the mycelium of the fungus. The test showed that there were no significant differences between the two isolates, Yu 4 and Yu 12. It was concluded that the intensive attack recorded in 1997 along the Danube River occurred due to exceptionally favorable environmental conditions for the development of the disease.