In this paper we shall prove a basic relation between the Frattini subgroup of the generalized free product of an amalgam
A
=
(
A
,
B
;
H
)
\mathfrak {A} = (A,B;H)
and the embedding of
A
\mathfrak {A}
into nonisomorphic groups, namely, if
A
\mathfrak {A}
can be embedded into two non-isomorphic groups
G
1
=
⟨
A
,
B
⟩
{G_1} = \langle A,B\rangle
and
G
2
=
⟨
A
,
B
⟩
{G_2} = \langle A,B\rangle
then the Frattini subgroup of
G
=
(
A
∗
B
)
H
G = {(A \ast B)_H}
is contained in
H
H
. We apply this result to various cases. In particular, we show that if
A
,
B
A,B
are locally solvable and
H
H
is infinite cyclic then
Φ
(
G
)
\Phi (G)
is contained in
H
H
.