We present radio and plasma wave science (RPWS) Langmuir probe (LP) observations that give evidence for a population of heavy, negative ions at altitudes below 900 km in Titan's ionosphere during the Cassini T70 flyby. The negative ion density in this region is comparable to, or higher than, the electron density of 760 cm−3. Both positive and negative ions are moving with a velocity of at least a few hundred m s−1 relative to Titan. We show two limiting cases where we have analysed RPWS/LP ion measurements. The data can be interpreted as either that a population of negative ions with density comparable to the electron density is present, moving at a very high (>2 km s−1) velocity, or that the ion population is moving at a few hundred m s−1, but with a density an order of magnitude larger than the electron density in the same region.