Compositions are disclosed herein comprising poly alpha-1,3-1,6-glucan with a weight average degree of polymerization (DPw) of at least 1000. This glucan polymer comprises at least 30% alpha-1,3 linkages and at least 30% alpha-1,6 linkages. Further disclosed are glucosyltransferase enzymes that synthesize poly alpha-1,3-1,6-glucan. Ether derivatives of poly alpha-1,3-1,6-glucan and methods of using such derivatives as viscosity modifiers are also disclosed.