Let p>1, : 0, and . # L (R) & L 1 (R) change its sign finite times. This paper is concerned with a Cauchy problemDefine the set of zeros of a solution u by Z(t)=[x # R : u(x, t)=0] for t>0. In the case of :=0, we show that the set Z(t) is contained in [&Ct, Ct] for large t>0 with some C>0 and that this order of t is best possible. When :>0, we also give estimates of Z(t) for global solutions and prove that Z(t)/[&K, K] for all t # (0, T ) with some K>0 for each blowup solution, where T is the blowup time.2001 Academic Press