Fifty-two Acacia koa trees with wilt/dieback disease symptoms in Hawaii were sampled for associated Fusarium spp. within their roots, stems and branches. Fusarium oxysporum was isolated from nearly 35% of all sampled roots and was especially common on healthy-appearing fine feeder roots. Fusarium solani was isolated most frequently from stem and branch tissues, but also occurred on more than 10% of sampled roots. A total of fourteen Fusarium spp. were detected within diseased trees; we suspect that most of these were probably non-pathogenic fungal endophytes. We found that F. oxysporum commonly colonized diseased trees, especially roots. Coupled with our results from pathogenicity testing, we believe that this fungal species is probably important in koa wilt/dieback disease etiology.