We examine the BPS and low energy non-BPS excitations of the D-string
in terms of open strings that travel on the D-string. We use this to s
tudy the energy thresholds for exciting a long D-string, for arbitrary
winding number. We also compute the leading correction to the entropy
from non-BPS states For a long D-string, and observe the relation of
all these quantities with the corresponding quantities for the element
ary string.