  Recently Cheng, Olinto, Schramm and Truran (COST) reexamined the constraints
from big bang nucleosynthesis (BBN) on the strength of primordial magnetic
fields. Their bottom line agreed with that of an earlier recent paper on the
subject (Kernan, Starkman and Vachaspati (KSV)), both in its final limit on the
magnetic field during BBN, and in its conclusion that for allowed values of the
magnetic field the dominant factor for BBN is the increased expansion rate at a
given temperature caused by the energy density of the magnetic field,
$B^2/8\pi$. However, their conclusion that weak interaction rates increased
with increasing B-field at these low field values contradicted the earlier
results of KSV. In this comment we point out that the Taylor series expansion
of the weak interaction rate about B=0 used in COST is not well-defined, while
the Euler-McLaurin expansion of KSV is well-behaved and reliable. Using the
Euler-McLaurin expansion we find that the weak interaction rates decrease
rather than increase with increasing B-field at small values of the B-field.
