Two cysteine residue containing merocytochalasans (cyschalasins A and B, <b>1</b> and <b>2</b>) and two 17,18-<i>seco</i>-aspochalasins (secochalasins A and B, <b>3</b> and <b>4</b>) were isolated from the endophytic fungus <i>Aspergillus micronesiensis</i>. Cyschalasins A and B represent a new type of merocytochalasan featuring the fusion of an aspochalasin with a modified cysteine residue. Secochalasins A and B are the first 17,18-<i>seco</i>-aspochalasins to be reported and represent a previously undescribed carbon skeleton. Plausible biosynthetic pathways of <b>1</b>-<b>4</b> were proposed. Compounds <b>1</b> and <b>2</b> were cytotoxic and active against Gram-positive bacteria.