Tomato yellow leaf curl bigeminivirus was isolated in Kuwait and therefore designated as TYLCV-K. The virus was found to be transmitted by whiteflies (Bemisia tabaci Genn.), and produced the characteristic external symptoms on tomato (Solanum lycopersicum L.) test plants. Electron microscopy of ultra-thin sections of infected leaves showed alterations in the ultrastructure of some organelles and uneven thickenings of the cell wall of the phloem tissue associated with severe damage in chloroplasts and abnormal building up of oily inclusions and empty vacuoles. The causal virus was characterized as a whitefly-transmitted monopartite Geminivirus. Viral genomic DNA fragment of 1.1 kb was successfully amplified from TYLCV-K infected tissues using polymerase chain reaction (PCR). The virus identity was confirmed by using Southern and dot-blot hybridizations. Chemical components of tomato leaves, such as photosynthetic pigments, especially chlorophyll a and b, total lipids, saturated and unsaturated fatty acids, reducing sugars and proteins, decreased in infected leaves when compared with healthy leaves of tomato test plants. On the other hand, total phenols accumulated to a greater extent in infected leaves. However, concentration of ascorbic acid (an antioxidant), leaf extract pH and total fatty acids were not dramatically affected by TYLCV-K. The level of increase or decrease of these constituents varied with different stages of plant growth (depending upon the developmental stages of the leaf at the time of infection). Our study showed that the level of some of the chemical constituents in tomato leaves changed in response to TYLCV-K infection.