Nymo thread is the most commonly used thread for Soutache. It is also ideal for beadstitching, beadweaving, and bead embroidery. It is slightly waxed, lightly twisted and is very strong. You need scissors to cut it and it may fray. To help keep the thread from fraying, you can condition the Nymo thread with Thread Heaven or other beading wax. This will also help cut down on the thread from knotting itself up while sewing and the conditioning will make weaving through the same bead several times a lot easier.
To use Nymo, stretch it well and "iron" it over a warm light bulb. When using mulitple colors, it is best to match the thread color to the lightest color in your work, or the thread may show. This is especially important when using Soutache.