Chemical investigation of three isolates of Penicillium crustosum Thom cultures, one of which was derived from a recent dog poisoning investigation, has led to the isolation and structure elucidation of secopenitrem D (1). Penitrems A-F and roquefortine C were also present in the isolates analyzed. The structure of 1 was unambiguously assigned based on extensive 1D- and 2D-NMR spectroscopic experiments, MS-aided structural studies and by comparison with structurally related congeners. Secopenitrem D lacks the C-16−C-18 ether linkage present in penitrems A-F.