Technical Abstract: Rambutan is an exotic tropical fruit that has increased in commercial importance for growers in Puerto Rico. In 2008 and 2009, fruit rot and lesions on both leaves and inflorescences were observed. A total of 276 diseased samples from these plant parts were collected at commercial orchards, Agricultural Experiment Stations of the University of Puerto Rico and the USDA-ARS in Mayaguez. Plant tissue was disinfested and plated on acidified potato dextrose agar (APDA) for fungal isolation. Besides other typical fungi normally associated with these tissue samples, Lasmenia sp. was also isolated and identified by morphology, morphometrics and PCR amplification of the ITS1-5.8S-ITS2 of the rDNA using primers ITS 1 and ITS 4 and the Large subunit (LSU) of the rDNA using primers Lr5 and LR0R. Lasmenia sp. was initially identified at the Fungal Biodiversity Centre (CBS) and confirmed by taxonomic keys in Puerto Rico. Five representative isolates (CBS 124122-124126) were deposited at the CBS. In APDA, colonies of Lasmenia sp. were cream-colored with dark brown concentric rings, and immersed, hyaline, branched and septate mycelium. Acervuli were produced on both APDA and plant tissue that was sampled from field and clean tissue that was inoculated with Lasmenia sp. Conidia, observed by scanning electron microscopy and light microscopy, were 10 to 12 µm long by 4 to 5 µm wide, light-brown, thick-walled, obclavate, aseptate and the apex was obtuse with a scar at the base. Pathogenicity tests were conducted on healthy, superficially sterilized fruits under laboratory conditions, on leaves from 6-month-old rambutan seedlings under greenhouse conditions, and on inflorescences at orchards. Either wounded or unwounded tissues were inoculated with a conidial suspension (2 to 4.5 x 106 conidia /ml) and 5-mm mycelial disks from each fungal isolate grown in APDA. Untreated controls were inoculated with APDA disks or sterile deionized-double-distilled water. Lasmenia sp. produced necrotic spots on leaves, rachis necrosis and flower abortion, fruit rot and water-soaked lesions on the fruit surface that spread to cause an aril rot. Acervuli were abundantly produced on fruit spintems (hair-like appendages). Koch’s postulates were fulfilled by reisolation of inoculated fungi from diseased tissue. A complete rDNA sequences for Lasmenia sp. were submitted to NCBI GenBank (Accession Nos. GU797405, GU797406, GU797407 and JF838336). Complete sequences of the LSU were submitted to GenBank (Accession Nos. JF838337, JF838338, JF838339, JF838340, and JF838341). Although there is a lack of DNA sequence data for the genus Lasmenia, which does not allow for confirmation of genus, sequences indicate a closer affinity to the Cryphonectriaceae (Diaporthales). Lasmenia sp. has been reported in Hawaii causing fruit rot in rambutan. To our knowledge, this is the first report of Lasmenia sp. causing, rachis necrosis and flower abortion worldwide, and the first report of Lasmenia sp. causing fruit rot and necrotic spots on leaves of rambutan in Puerto Rico.