All sequenced angiosperm mitochondrial genomes contain plastid-derived DNA, usually between 5 kb and 25 kb of these foreign sequences and sometimes over 100 kb. However, little is known about the tempo and pattern of gain and loss of plastid DNA in angiosperm mitochondrial genomes. To address this issue, we devised a computational approach to estimate rates and sizes of plastid insertions and deletions in a phylogenetic context and applied it to several groups of closely related mitochondrial genomes. Of particular interest are seven mitochondrial genomes in Monsonia (Geraniaceae). We estimate that across this group, some 400 plastid-DNA insertion events have occurred, many of them within the past million years. Rates of plastid DNA insertion and deletion each vary significantly over time in this group and independently of one another. Overall, the rate of gain of new nucleotides per site that can be attributed to plastid DNA insertion is more than an order of magnitude higher than the synonymous substitution rate in Monsonia mitochondrial DNA. Analysis of mitochondrial genomes in other angiosperm groups suggests similarly high rates of ongoing plastid DNA insertion. These findings decidedly reject the published claim that most plastid-derived sequences in angiosperm mitochondrial genomes are the result of relatively few and ancient transfer events.