Microscopic fungi move around the world, unnoticed in many cases, on their plant hosts. Interceptions of fungi on plants from cargo and passengers provide a snapshot on the movement of little-known fungi on understudied hosts and reveal information on the global distribution of these fungi. Samples from passive domestic surveillance likewise provide an opportunity to record and confirm the presence of poorly documented species. The USDA U. S. National Fungus Collections (BPI) house over 19000 specimens of fungi that were either intercepted at ports of entry or submitted to the USDA from domestic surveillance. These collections, dating back to 1863, provide an invaluable resource not only for taxonomic research but also for biosecurity pest identification activities. Several case studies will be presented highlighting the importance of collections for the identification of unknown fungi, especially those that are under-reported and which may be represented by as few as one voucher specimen. For example, a historical powdery mildew specimen collected in California in the 19th century, never formally described, was used to identify a fungus found 120 years later, also in California, showing that the mildew was endemic, avoiding destruction of an entire greenhouse of a new ornamental hybrid. In another example, an ascomycete on the herbaceous plant Commelina (dayflower) was intercepted in Florida, USA, arriving from the Carribean. A fungus with the same morphology was described in Taiwan in 1943 as Physalospora commelina. An isotype collection, one of only two known specimens, is held at BPI. Examination of the type showed morphological discrepancies with the original description, but it was consistent with the intercepted fungus and allowed for positive identification of the intercepted sample. Deposition of samples, even those with limited material, into curated collections continues to be important to not only support taxonomic research but also day-to-day biosecurity activities.