Aspergillus fumigatus, a common feed contaminant particularly ubiquitous in conserved forages, produces several mycotoxins that can affect the health of animals. The aim of this work was to assess the ability of A. fumigatus to produce toxins, particularly gliotoxin, on laboratory media and natural feed substrates. The ability of fourteen A. fumigatus strains to produce gliotoxin, verruculogen, fumagillin, and helvolic acid was evaluated on defined medium. Mycotoxin production was widespread; gliotoxin and verruculogen were produced by 70 and 85% of isolates at concentrations up to 62.2 and 3.5 μg mL−1, respectively. In addition, all strains were positive for fumagillin (up to 25.9 μg mL−1) and helvolic acid (up to 3.5 μg mL−1). Growth and gliotoxin production of a gliotoxin-positive isolate was evaluated on several common animal feeds. Growth was extensive on cereals—wheat, corn, barley, and triticale, less pronounced on forage grasses—rye grass, orchard grass, and tall fescue, and negative or scarce growth was observed on leguminous forages—alfalfa and red clover. Gliotoxin production was in general correlated to growth, except for corn and tall fescue. In these two substrates, as well as in leguminous forages the toxin was not detected. Low pH culture conditions, similar to those present in silages, reduced gliotoxin production as compared to controls although growth was unaffected. Concentrations found in cereals and grasses were up to 17.5 and 1.5 μg g−1, respectively. The stability of A. fumigatus mycotoxins during storage was evaluated in orchard grass and ryegrass. Toxins were stable in both forages, except for fumagillin whose concentration decreased rapidly. After eight weeks of storage, the amount of gliotoxin, verruculogen, helvolic acid, and fumagillin was 63, 76, 89, and 10% of the initial concentration, respectively. The high concentrations found in experimentally contaminated feed and the relative stability of A. fumigatus toxins could pose a potential health risk for animals consuming contaminated feeds.