Abstract: A cornerstone result in extremal graph theory is Mantel's Theorem, which states that every maximum triangle-free subgraph of $K_n$ is bipartite. A sparse version of Mantel's Theorem is that, for sufficiently large $p$, every maximum triangle-free subgraph of $G(n,p)$ is w.h.p. bipartite. Recently, DeMarco and Kahn proved this for $p > K\sqrt{\log n/n}$ for some constant $K$, and apart from the value of the constant, this bound is best possible. We study an extremal problem of this type in random hypergraphs. Denote by $F_5$ the 3-uniform hypergraph with vertex set $\{a,b,c,d,e\}$ and edge set $\{abc, ade, bde\}$. Frankl and Füredi proved that the maximum 3-uniform hypergraph on $n$ vertices containing no copy of $F_5$ is tripartite for $n>3000$. A natural question is that for what $p$ is every maximum $F_5$-free subhypergraph of $G^3(n,p)$ w.h.p. tripartite. We show this holds for $p>K \log n/n$ for some constant $K$ and does not hold if $p=0.1\sqrt{\log n}/n$. Joint work with Jozsef Balogh, Jane Butterfield and John Lenz.