Understanding the properties of beneficial mutations is essential for a better understanding of adaptive evolution, since these mutations are the fuel of adaptation. Not only their individual fitness properties are important, but also their combined effect on fitness, which may deviate from the additive or multiplicative expectation that is often assumed in population genetic models. For instance, if the sign of the fitness effect of mutations (i.e. positive or negative) depends on the presence of other mutations, called sign epistasis, such interactions present an adaptive landscape with multiple fitness peaks, where the number of mutational pathways leading to higher fitness is constrained. Yet, empirical studies of beneficial mutations and their interactions are scarce, because beneficial mutations are rare and often inaccessible to experimental study, particularly the study of their interactions. Here, I propose to study the properties of beneficial mutations in experiments with the filamentous fungus Aspergillus nidulans. Using previously developed protocols to isolate beneficial mutations and to construct mutants with all possible combinations of beneficial mutations, we seek to study how selected mutations at the same (dominance) and different loci (epistasis) interact in determining fitness. The resulting data set will be one of the first presenting direct empirical information on the topography of the fitness landscape underlying adaptation.