Dynamic viscoelasticity and biaxial extensional flow behavior of polystyrene melts with various molecular weight distributions have been studied in wide ranges of temperature and strain rate epsilon(B). Two patterns of time dependence of the biaxial stress growth coefficient eta(B)(+) are Observed with increasing reduced strain rate epsilon(B) tau(0), where tau(0) is a characteristic relaxation time. At short times, eta(B)(+) coincides with the linear viscoelastic curve; then it shows strain hardening after that, or strain softening followed by a quasisteady state. These behaviors are observed for polydisperse polystyrenes at much higher epsilon(B) tau(0) than those for monodisperse polystyrenes. The quasisteady-state viscosity eta(B,qs) decreases with the strain rate. Addition of a very high molecular weight component causes considerable effects on tau(0) and on the strain rate dependences of shear eta* and biaxial eta(B,qs) viscosities as well as on the stress growth coefficient eta(B)(+), where eta* is the absolute value of the complex viscosity. The relaxation time 70 increases very much, and the strain rate dependences of eta* and eta(B,qs) become much weaker. The upturn tendency of eta(B)(+) becomes prominent when B compared at the same reduced strain rate epsilon(B) tau(0). On the other hand, when compared at the same value of the reduced viscosity eta(B,qs)/6 eta(0), a similar degree of strain softening in eta(B)(+) is observed for all polydisperse samples, where eta(0) is the zero shear viscosity.