A singular masa $A$ in a ${\mathrm{II}}_1$ factor $N$ is
defined by the property that any unitary $w\in N$ for which
$A=wAw^*$ must lie in $A$. A strongly singular masa $A$ is one
that satisfies the inequality
¶ \[ \|\bb E_A-\bb
E_{wAw^*}\|_{\infty,2}\geq\|w-\bb E_A(w)\|_2 \]
¶ for all
unitaries $w\in N$, where $\bb E_A$ is the conditional
expectation of $N$ onto $A$, and $\|\cdot\|_{\infty,2}$ is
defined for bounded maps $\phi :N\to N$ by
$\sup\{\|\phi(x)\|_2:x\in N,\ \|x\|\leq 1\}$. Strong
singularity easily implies singularity, and the main result of
this paper shows the reverse implication.