The molybdenum-based monoaryloxide monopyrrolide (MAP) species, Mo(NAd)(CHCMe2Ph)(C4H4N)(HIPTO) (2a), which contains "small" imido (Ad = 1-adamantyl) and "large" aryloxide (HIPTO = O-2,6(2,4,6-i-Pr3C6H2)C6H3) ligands, catalyzes Z-selective metathesis reactions as a consequence of intermediate metallacyclobutane species not being able to have an (anti) substituent pointing toward the HIPTO group. Ring-opening metathesis polymerization (ROMP) of dicarbomethoxynorbornadiene (DCMNBD) with 2% 2a in toluene leads to >99% cis and >99% syndiotactic poly(DCMNBD), white ROMP of cyclooctene and 1,5-cyclooctadiene (300 equiv) with initiator 2a leads to poly(cyclooctene), and poly(cyclooctadiene) that have cis contents of >99%; all are previously, unknown microstructures. Z-Selectivity is also-observed in the metathesis of cis-4-octene and cis-3-hexene by initiator 2a to give cis-3-heptene.