Given the solution f of the sequential fractional differential equation

_{a}D^{α}_{t} (\ _{a}D^{α}_{t} f ) + P( t ) f = 0, t ∈ [b, c],\ \ where\ \ -$\infty$ \< a \< b \< c \< +$\infty$, α ∈ (1/ 2 , 1)\ \ and\ \ P : [a, +$\infty$) {\textrightarrow} [ 0, P_{$\infty$}], P_{$\infty$} \< +$\infty$,\ \ is continuous. Assume that there exist\ \ t_{1}, t_{2} ∈ [b, c]\ such that\ f( t_{1\ }) = (_{a}D^{α}_{t }f )( t_{2\ }) = 0. Then, we establish here a positive lower bound for c - a which depends solely on α, P_{$\infty$}. Such a result might be useful in discussing disconjugate fractional differential equations and fractional interpolation, similarly to the case of (integer order) ordinary differential equations.