Three related new alkylphenols, termed anaephenes A-C (1-3), containing different side chains, were isolated from an undescribed filamentous cyanobacterium (VPG 16-59) collected in Guam. Our 16S rDNA sequencing efforts indicated that VPG 16-59 is a member of the marine genus Hormoscilla (Oscillatoriales). The structures of anaephenes A-C (1-3) were elucidated by spectroscopic methods, and compounds assayed for growth inhibitory activity against prokaryotic and eukaryotic cell lines. Anaephene B (2), possessing a terminal alkyne, displayed moderate activity against B. cereus and S. aureus with MIC values of 6.1 μg/mL. While 1 and 3 showed no pronounced activity in these assays, their structural features highlight the unusual biosynthetic capacity of this cyanobacterium and warrant further study.