Source code for the extension is at http://www.thuban.org/projects (see AstroMath). As suggested, I set up a couple threading/non-threading tests outside of Zope. Here are the results, and the sourcecode for the scripts. My guess now is that it's a Python error, or something that I don't know about how C-extensions behave under Python threads. AstroMathTest.py -- threaded, import outside of the thread run() method AstroMathTest-2.py -- threaded, import inside the thread run() method AstroMathTest-3.py -- unthreaded, import outside of the class AstroMathTest.py triggers an abnormal termination (when running under Zope, this crashes the server). The others execute normally. ---------- Keith J. Farmer kfarmer@thuban.org http://www.thuban.org D:\Users\kfarmer\Desktop>AstroMathTest.py abnormal program termination D:\Users\kfarmer\Desktop>AstroMathTest-2.py Period: 2448976 - 2448982 Body: 2 N: 1000 Results: [(-10239.349040235875, 1251.1652772821858, -5496.6220548625606, 51274.9 53459961718, -206301.84922582778, 460502.53941229283), (3.2679109787377607, -135 .6962193181536, -199.45658237327001, 12202.821401570927, -157124.72908036769, 12 05113.4818641837), (0.41752859359838751, 1.7776704736485571, -12.673879235861317 , -106.15373488272769, 1133.234164492935, -6902.3318907419343)] Period: 2448976 - 2449067 Body: 9 N: 1000 Results: [(287.98364420672812, 4.1126841002230572, -0.011082571365048428, -0.002 8990940486140681, 0.0091456171007818048, -0.0023508733523008038), (-0.4314454132 874721, -0.045976323069640314, 0.0012300667332541073, 1.4911799720493891e-005, - 6.6414232785952655e-005, 0.00012924994264322819), (19.570152597202206, 0.0619786 88999307245, -0.00097406579798330561, 0.00090494391238523405, -0.003583331057906 0948, 0.0047379229395861609)] Period: 2448976 - 2449010 Body: 2 N: 1000 Results: [(-10239.348643429543, 1251.003681396081, -5483.5515126184919, 51109.86 3896315524, -219235.21057991375, 810132.71983693726), (3.2689078168908492, -136. 20534786325385, -136.04987455483274, 8924.1019692440859, -75505.413571723024, 34 4372.13770327822), (0.41751500632241678, 1.7840128152238572, -13.381959141681486 , -74.062877111367413, 445.92397716704875, -665.34152725392778)] All threads completed D:\Users\kfarmer\Desktop>AstroMathTest-3.py Period: 2448976 - 2449003 Body: 2 N: 1000 Results: [(-10239.348975676005, 1251.1425823831078, -5496.7888457231365, 51584.1 48366467409, -226341.59545085297, 847763.8068514691), (3.268369386507203, -135.9 7655469686643, -158.19119069105136, 9729.4932483937864, -87754.834019993446, 410 204.00677085714), (0.4175246669475709, 1.7799845370127327, -12.999161326210897, -87.738413865454802, 650.23756109944247, -1744.1836844830825)] Period: 2448976 - 2448982 Body: 6 N: 1000 Results: [(-981.43530705578974, 174.81971790840851, -43.979617112478337, 39.7926 31131402359, 9.9104070099232775, 6.2143370765762951), (1.3964600969705743, 3.703 8277998777773, -7.4353275545974817, -1.6166923220752221, 6.2503824630868579, -7. 9853824343306314), (1.5903224291707663, 0.40258974548554582, -0.3925627801526973 5, -0.55855403338606679, 0.48021519119362804, 0.38498396222805825)] Period: 2448976 - 2449075 Body: 2 N: 1000 Results: [(-10243.831880826461, 1825.1908096745419, -21196.163255795575, 192251. 50787759182, -570528.83454192919, 482519.3051117654), (1.1206641242181234, 151.4 5981349096382, -8502.2169261519302, 92353.897615713213, -355123.92413740553, 449 769.85125815234), (0.42448055258943779, 0.58474259832985709, 32.255979151714399, -701.70688036347042, 3828.7795007674963, -6288.2151422017569)] All threads completed