Disclosed herein are compositions comprising insoluble alpha-glucan particles having a high degree of crystallinity and small particle size. For example, the alpha-glucan particles can have a degree of crystallinity of at least about 0.65, and/or an average size of less than a micron. At least 50% of the glycosidic linkages of the insoluble alpha-glucan in the disclosed particles are alpha-1,3 glycosidic linkages. Further disclosed are methods of producing insoluble alpha-glucan particles, as well as their use in various applications and products.