Likelihood under Type I censoring

Let $X_1,\dots,X_n$ be a random sample from a lifetime distribution $F$ with parameter $\theta$, density $f$, and let $T_0$ be a fixed time. We observe $X_i=x_i$ only if $x_i\le T_0$. Let $n_u$ be the number of observed lifetimes. Which is the most appropriate `likelihood' for estimating $\theta$?