  The flex divisor of a primitively polarized K3 surface $(X,L)$ of degree
$L^2=2d$ is, generically, the locus of all points $x\in X$ for which there
exists a pencil $V\subset |L|$ whose base locus is $\{x\}$. We show that the
flex divisor lies in the linear system $|n_dL|$ where $n_d=(2d+1)C(d)^2$ and
$C(d)$ is the Catalan number. We also show that there is a well-defined notion
of flex divisor over the whole moduli space $F_{2d}$ of polarized K3 surfaces.
