Invasive species attracted great attention due to theirability to affect ecosystem integrity. Assessment of zebra mussel density, mortality and their infestation to unionids was carried out at four sites of lake Balaton in April 2010. The relative abundance of zebra mussels was eminently exceeding those of unionids. High incidence of unionids infested with zebra mussels was reported at Balatonakali (100%), Balatonboglár (93.33 %), Balatonalmádi (66.67%) and Keszthely (43.39 %). The percentage of collected dead unionids was the highest at Balatonboglár (40 %), followed by Balatonalmádi (33.33 %) and was relatively low at Balatonakali (9.09%) and Keszthely (8.16 %). The average numbers of zebra mussels colonizing each unionids were 30±3.019, 23.0±1.411, 19.818±5.088 and 3.0±0.577 at Balatonboglár, Keszthely, Balatonakali and Balatonalmádi, respectively. Percentage of zebra mussel mass/ unionid mass is given in a decreasing order, showing high unionid infestation intensity at Balatonboglár (35.512±5.432%) and Balatonalmádi (20.191±1.638 %) whilst relatively low infestation intensity was reported for unionids at Balatonakali and Keszthely. The percentage of zebra mussel mass/ unionid mass was strictly correlated with the incidence of dead unionids at the tested sites. This might indicate that zebra mussel invasion in Lake Balaton poses actual risk on unionid mussel populations.