We report an examination of the ring-opening metathesis polymerization (ROMP) of endo-dicyclopentadiene (DCPD) by 10 well-defined molybdenum-based and 16 tungsten-based alkylidene initiators. Five tungsten-based MAP (monoaryloxide pyrrolide) initiators with the general formula W(X)(CHCMe2Ph)(Me(2)Pyr)(OAr) (X = arylimido, alkylimido, or oxo; Me(2)Pyr =2,5-dimethylpyrrolide; OAr = an aryloxide) were found to yield >98% cis, >98% syndiotactic poly(DCPD); they are W(N-t-Bu)(CHCMe3)(pyr)(OHMT) (2, OHMT = O-2,6-(2,4,6-Me3C6H2)(2)C6H3, pyr = pyrrolide), W(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(pyr)(OHMT) (3), W(O)(CHCMe2Ph)(Me(2)Pyr)(OHMT)(PPh2Me) (7, Me2Pyr =2,5-dimethylpyrrolide), W(O)(CHCMe2Ph)(Me(2)Pyr)(ODFT)(PPh2Me) (9, ODFT = O-2,6-(C6F5)(2)C6H3), and W(O)(CHCMe2Ph)(Me(2)Pyr)(OTPP)(PMePh2) (10, OTPP = O-2,3,5,6-Ph4C6H). Two biphenolate alkylidene complexes, Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(rac-biphen) (17) and W(N-2,6-Me2C6H3)(CHCMe2Ph)(rac-biphen) (22, biphen =3,3'-(t-Bu)(2)-5,5'-6,6'-(CH3)4-1,1'-biphenyl-2,2'-diolate), were found to yield >98% cis, >98% isotactic poly(DCPD). Cis, syndiotactic or cis, isotactic poly(DCPD)s (made with 50-1000 equiv of DCPD) are accessible within seconds to minutes in dichloromethane at room temperature. No isomerization or cross-linking reactions are observed, and addition of a chain transfer reagent (1-hexene) or the use of THF as a solvent does not decrease the stereospecificity of the polymerizations. Cis, syndiotactic and cis, isotactic poly(DCPD)s can be distinguished readily from each other by C-13 NMR spectroscopy. Hydrogenation of each stereoregular poly(DCPD) produces H-poly(DCPD)s that have melting points near 270 degrees C (syndiotactic) or 290 degrees C (isotactic) and high crystallinities (w(c) = 0.83 for syndiotactic and wc = 0.74 for isotactic).