Abstract: Milarite was found in a cavity of a small pegmatitic segregation in association with albite, stilpnomelane and chabazite. Hexagonal, well-shaped crystals of milarite up to 4 mm in length are transparent, pale green in colour, and show bright green cathodolumin-escence. The refractive indices are: ω = 1.535, ε = 1.534(Na). D = 2.55 g cm⁻³. The unit cell dimensions are: a 10.418(4), c 13.817(7) Å, V 1298.7 ų. Anomalous biaxial sectors and birefringent optical patterns are visible in basal section. 2V varies from 34° in the {0001} sector to 64° in {10¯11} sector. OAP azimuths are related to the external hexagonal symmetry of the crystals. Microprobe analyses of sections perpendicular and parallel to the c-axis revealed a uniform distribution of alkalis, mainly K₂O, whereas CaO and Al₂O₃ contents are slightly higher in prismatic and pyramidal sectors than in the basal sector, but no systematic chemical zoning can be distinguished. The Al₂O₃ content decreases in the outer zone of the crystal studied due to Be-Al substitution. The average ‘anhydrous’ chemical formaula of the Strzegom milarite is: (K_0.98Na_0.01)Ca_1.93Mn_0.02(Be_2.16Al_0.93)(Si₁₂O₃₀). Infra-red data indicate the presence of H₂O molecules in the milarite. Internal growth structures of the crystals indicate rhythmic fluctuations of the growth rate resulting in an internal strain, inducing anomalous optical patterns. HRTEM study did not reveal any significant distortion of the milarite lattice.