We prove that for N = 6 and N = 10, there do not exist any non-zero semistable abelian varieties over Q with good reduction outside primes dividing N . Our results are contingent on the GRH discriminant bounds of Odlyzko. Combined with recent results of Brumer-Kramer and of Schoof, this result is best possible: if N is squarefree, there exists a non-zero semistable abelian variety over Q with good reduction outside primes dividing N precisely when N / ∈ {1, 2, 3, 5, 6, 7, 10, 13}. 1