Quinone outside inhibiting fungicides (QoIs) represent one of the most important fungicide classes used to control Plasmopara viticola, the causal agent of downy mildew in grapevine. Soon after the introduction of QoI fungicides in vineyard, resistant isolates have been detected. To manage resistance evolution, it is important to understand how resistant populations emerge, spread and survive. One major approach in understanding these processes is to measure the pathogen fitness in the absence of fungicide pressure and to investigate the competitiveness of resistant isolates. In this study, different isolates of P. viticola were included to measure the Composite Index of Fitness. It was also investigated whether there is a fitness cost in QoI-resistant isolates. No fitness cost was detected for the tested isolates under controlled conditions suggesting that only highly fit isolates are selected under field conditions leading to a balance between sensitive and resistant isolates.