Author: Dr. M. Hertweck, Mathematisches Institut B, Universit\"at Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany Title: Units of $p$-power order in principal blocks of $p$-constrained groups Abstract: Let $G$ be a finite group which has a normal $p$-subgroup $N$ with $\cen{G}{N}\leq N$, and let $R$ be a $p$-adic ring. Let $\NU{RG}$ denote the group of units in $RG$ of augmentation $1$. It is shown that any finite $p$-group in $\NU{RG}$ which normalizes $N$ is conjugate to a subgroup of $G$ in the units of $RG$, and that any finite $p$-group in $\NU{RG}$ which centralizes $N$ is contained in $N$. Current status: submitted for publication