Microsteris is a monotypic genus of flowering plants in the phlox family containing the single species Microsteris gracilis, known by the common name slender phlox.
Microsteris gracilis is native to western North America from northwestern Canada to the American Midwest and West Coast, through Mexico, as well as parts of South America. It is native to all the diverse plant communities in California. 
Microsteris gracilis is an annual herb which is variable in shape, taking a decumbent, branching, sometimes almost tuftlike form or growing erect and very slender. Its maximum height approaches 20 centimeters, but it may be much smaller.
The lance-shaped leaves are 1 to 3 centimeters long and oppositely arranged except for the upper ones, which are alternate. The herbage is glandular and hairy in texture. The inflorescence at the top of the stem bears one or more small flowers.
The flower has a tubular throat around a centimeter long encased in a tubular calyx of sepals. The flat corolla has five flat-tipped or notched lobes just 1 or 2 millimeters long. The flower is white to bright pink with a yellowish throat.
- Prather, L. A., C. J. Ferguson, and R. K. Jansen. (2000). Polemoniaceae phylogeny and classification: implications of sequence data from the chloroplast gene ndhF. American Journal of Botany 87 1300-08.
- Ferguson, C. J. and R. K. Jansen. (2002). A chloroplast DNA phylogeny of eastern Phlox (Polemoniaceae): implications of congruence and incongruence with the ITS phylogeny. American Journal of Botany 89 1324-35.
- Jepson eFlora: Distribution map for Microsteris gracilis in California
- Jepson Manual Treatment of Microsteris gracilis
- USDA Plants Profile for Microsteris gracilis (slender phlox)
- Microsteris gracilis — U.C. Photo gallery
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