malthusian growth statistics

I've got populations consisting of a mix of two strains. I have the numbers of each strain at t=0hrs and t=24hrs. I have been trying to calculate the Malthusian growth parameter for each replicate. I am using the formula ln (Nf / N0) where Nf is the final number, and N0 is the starting number. In some of my replicates the final number of one of the strains is 0. ln(0) is giving me some trouble in excel, as I think that ln(0) is infinity, or doesn't exist. Can anyone shed some light on how one goes about doing these sorts of statistics on these sorts of data?

Many thanks