Adaptations are constructed through the sequential substitution of beneficial mutations by natural selection. However the rarity of beneficial mutations has precluded efforts to describe even their most basic properties. Do beneficial mutations typically confer small or large fitness gains? Are their fitness effects environment-specific, or are they broadly beneficial across a range of environments? To answer these questions we used two subsets (n = 18 and n = 63) of a large library of mutants carrying antibiotic resistance mutations in the bacterium Pseudomonas fluorescens whose fitness, along with the antibiotic sensitive ancestor, were assayed across 95 novel environments differing in the carbon source available for growth. We explore patterns of genotype by environment (GxE) interactions and ecological specialization among the 18 mutants initially found superior to the sensitive ancestor in one environment. We find that GxE is remarkably similar between the two sets of mutants and that beneficial mutants are not typically associated with large costs of adaptation. Fitness effects among beneficial mutants depart from a strict exponential distribution: they assume a variety of shapes that are often roughly L shaped but always right truncated. Distributions of (beneficial) fitness effects predicted by a landscape model assuming multiple traits underlying fitness and a single optimum provide often a good description of the empirical distributions in our data. Simulations of datasets containing a mixture of single- and double-mutants under this landscape show that inferences about the distribution of fitness effects of beneficial mutants is quite robust to contamination by second site mutations.