We report the synthesis and morphological characterization of a miktoarm block copolymer architecture : (PSalpha M-PIM)(n)-(PSM-PIalpha M)(n), where M similar to 20 000, n = 1, 2, and the arm asymmetry parameter alpha = 1, 2, or 4 (alpha is the ratio of the outer block molecular weight to that of the inner block). These block copolymers are symmetric in overall composition and exhibit n- and alpha-dependent microdomain morphologies. Alternating lamellae are observed for linear tetrablocks (n = 1), alpha = 1, 2, 4, and for inverse starblock (n = 2), alpha = 1, 2. An architecturally-induced morphological transition from lamellae to a tricontinuous cubic structure is observed with n = 2 and alpha = 4. The formation of the tricontinuous cubic microdomain structure in this compositionally symmetric system is thought to relieve the overcrowding of the four peripheral PS-PI junctions by providing a curved intermaterial dividing surface with a triply periodic microdomain structure, allowing some bridging by the interior blocks of the miktoarm star.