In triangle \( ABC \), we know that angle \( A \) is \( 90^\circ \) and angle \( C \) is \( 34^\circ \). To find angle \( B \), we can use the fact that the angles in a triangle sum up to \( 180^\circ \).
So, we can find \( m \angle B \) using the equation:
\[ m \angle A + m \angle B + m \angle C = 180^\circ \] Substituting the known values:
\[ 90^\circ + m \angle B + 34^\circ = 180^\circ \]
Combining the known angles:
\[ 124^\circ + m \angle B = 180^\circ \]
Now, subtract \( 124^\circ \) from both sides:
\[ m \angle B = 180^\circ - 124^\circ = 56^\circ \]
Thus, the measure of angle \( B \) in triangle \( ABC \) is:
\[ \boxed{56^\circ} \]